Asked in a 70th birthday interview whether he would carry on "meddling" in politics when he became King, the then Prince of Wales was unusually blunt. "I'm not that stupid," he replied.
He cited Shakespeare as he explained how heirs have to change when they become the sovereign, saying that Henry V or Henry IV parts I and II show that newly-crowned monarchs have to "play the role in the way that it is expected".
No one who knows Charles, however, expects him to reign in the same way as his mother, Queen Elizabeth II.
"There is plenty of freedom for each monarch to do different things in their own way," said one former courtier. "We think there is only one way for it to be done because we have only ever seen one monarch in most of our lifetimes so we can't remember anyone else doing it another way."
The new King, unlike his mother or grandmother, grew up knowing he was destined to sit on the throne, and has had an entire lifetime to think about how he will go about it.
The extent to which King Charles III will throw his weight around in the political sphere, typified in the past by his "black spider" memos to government ministers, is the most obvious question about his reign, but there are plenty of other issues that will define him.
Will he "slim down" the monarchy by ditching some of the fringe players who carry out official duties? What will he do with the multiplicity of grand homes he has inherited, including Balmoral and Sandringham, not to mention official residences like Buckingham Palace? What will happen to his high-profile charities? And what about the Commonwealth?
One of the most obvious and immediate differences between the Queen and her son, according to those who have worked with him, will be in tone.
The Queen always maintained a certain dignified distance from her subjects, as was the norm when she came to the throne, making her a figure of mystique as well as majesty.
Charles, however, is far more of a known quantity. We know his strengths and weaknesses, his triumphs and his betrayals, making him more flawed, but also more accessible.
"He has lived through the decline of deference," said one ally. "He has been much more open than the Queen because he comes from a generation that has been much more open. We will see his humanity and his humour, as we have always done."
He will find much greater pressures on his time than when he was Prince of Wales, chiefly from the red boxes of government papers that come in every day, from the audiences with ambassadors and dignitaries, and from the endless duties he must perform as monarch, none of which involve expressing controversial opinions.
While the King might have to tone down his public interventions, he is not about to stop fighting for the causes he has championed for decades, chiefly the natural world and the built environment.
Penny Junor, a biographer who has spent decades chronicling the life of the new King, said: "My suspicion is that he won't meddle in an obvious way, but he will quite legitimately talk to his Prime Minister in a more opinionated way than his mother.
"He is who he is, and he has some firm views which are based on knowledge. He is an incredibly knowledgeable man. He has spoken to the experts in all these fields he is interested in.
"To expect him to have a Prime Minister who comes in and talks to him about agriculture or climate change, he is not going to just sit there and say 'would you like another cup of tea'. But I don't think he will be writing black spider memos."
Those who have worked closely with him agree. "He has never said he will suddenly stop caring about those issues when he becomes King but he does understand the way he addresses and campaigns on those issues will have to change," said one.
When he was asked by Sir Trevor McDonald in 2006 whether he acknowledged that some people saw him as a "nuisance", he was unapologetic, saying: "I mind deeply about this country and the people here."
He added that it would be negligent of him, given the opportunity that his position affords him, not to try to solve problems.
One friend of the King said: "He has always collected interesting people and he won't stop doing that. He invites people to Sandringham for beach weekends. He could use lunches at Buckingham Palace to get people together. He has tremendous convening power and he will keep on using it."
Those who have worked for the King scoff at critics who have in the past tried to suggest he is somehow lazy. They speak of a man who is constantly on the go, who fills his days with engagements and then retires to his study to work late into the night, often on matters that are entirely removed from public view.
"He has got a sensitivity to him that gets underplayed," said one. "He is an empathetic person. Often people say they have received the most wonderful letters from him. If there is a disaster of some sort he will write personally to all the people who have been affected by it, but he wouldn't expect or want any thanks or glory for that. He would just see it as a decent thing to do."
The Duke of Sussex said of him in one interview: "He does need to slow down, this is a man who has dinner ridiculously late at night.
"And then goes to his desk later that night and will fall asleep on his notes to the point of where he'll wake up with a piece of paper stuck to his face."
He has not, however, spent the past 20 years itching to start the job he was born to do, according to those who have served him.
One said: "The idea that he has been waiting all these years to become king is not really correct because he has always been squeamish about anything relating to succession planning because it meant thinking about his mother dying and he didn't want to go there.
"By the time his father died all of his siblings were on their third or fourth time filming their contributions [to television obituaries], but he had never done it and only agreed to it much nearer the time. He is a very sensitive soul which is why he was so profoundly hurt by what Harry and Meghan said when they left the country."
There will be fewer invitations, however, for minor members of the Royal family, as the new King gradually tries to make the institution leaner and cheaper for the taxpayer.
He is acutely aware that he is often portrayed as extravagant, sometimes fairly, but he has made it clear that the monarchy should not be "unnecessarily costly", according to one well-placed source.
"He's not someone who throws money around," the source said. "He repairs and recycles his clothes and when he eats at home it's an omelette or a baked potato.
"You won't see the balcony filled with publicly-funded members of the Royal family any more."
One idea he has raised with courtiers is to make Buckingham Palace more available to the public, perhaps by opening it for more of the year, or by allowing more commercial events to take place in order to help balance the books. "He doesn't feel he needs the whole building," said one former aide.
Indeed the wider issue of what happens to all of the state-owned palaces and privately-owned homes he has inherited is one of the more intriguing aspects of this new reign.
One friend said: "His home, if you ask him, is Birkhall, all day long - even more than Highgrove. He normally lives in his study and his bedroom and everything else is staff and great rooms. But he recognises the importance and symbolism of living at Buckingham Palace because that is Royal HQ."
As Penny Junor puts it: "He has said that if he was living in a two-up, two-down no-one would come to lunch, but because he is living in a palace people will come. He has been able to get heads of corporations whose diaries are choc-full, but they will make time for him and he sends them away with homework to solve problems."
An idea that has taken root in royal circles is that Balmoral, a royal residence since the estate was bought by Prince Albert in 1852, could be given away by Charles, while he retains Birkhall on the same estate.
One courtier said: "If you look at Highgrove, Birkhall and Clarence House they are grand houses rather than palaces.
"What does that mean in terms of him hanging on to massive buildings? He has gifted Dumfries House to the Scottish nation. What that means for some of those other buildings, I don't know, but you can see the logic."
Highgrove could be passed down to Prince George, Sandringham could become the new Norfolk base for William and Kate - now the Duke and Duchess of Cornwall - and Windsor Castle a weekend home for the whole family.
Regular "board meetings" with his son and heir will be a key feature of his reign.
One friend said: "He will work very much in partnership with William because any long-term decisions he makes are going to reach George probably, and one of the things that has brought him closer in recent years is the consultative relationship they have. It will be a two-way conversation."
It is easy to forget that the Queen stopped travelling abroad in 2015 (after flying a combined total of more than one million miles), so the resumption of State visits to foreign countries by the new King will be part of the process of renewal.
He is likely to be far more visible generally, taking every opportunity to get out and meet the public. "He is out and about almost every single day and he loves it," said one aide.
His State visits are likely to focus on Commonwealth countries, as Charles "has a very keen eye" on the institution, according to one courtier. "It is a grouping of nations with a very young demographic and he knows that if you set them the right challenges you can drive out solutions."
He will not, however, voice objections if any Commonwealth realms decide to become republics and replace him as their head of state - a constant question in Australia, New Zealand, Jamaica and other realms.
As one friend said: "He is much more relaxed than people realise about how the realms will respond and if they decide to change I can't see him agonising over that.
"Knowing him I think he would absolutely recognise the view that it's a bit ridiculous to have your head of state living on the other side of the world. He wouldn't want to be part of some huge shouting match."
Much has been made of the King's desire, as head of the Church of England, to be known as "defender of faith" rather than "defender of the faith", a title that every monarch has inherited from Henry VIII.
As part of the accession he will have a meeting with leaders of all faiths, something which certainly did not happen in 1952 when the Queen ascended the throne.
He is "pretty sure he will offend some people", those close to him say, "but his position is that everyone should have a freedom and a right to pursue faith, and it would be supported by the Church of England and every major cleric and creed".
It is not only Charles's role that has changed, of course. The Duchess of Cornwall is now Queen Consort, and it will be her job not only to offer him moral and emotional support, but also to speak plainly to power, particularly when it comes to the less effective members of his entourage.
"There are people who get close to him who he thinks can help him but who are not always the best people to be around him, and that's perhaps his biggest vulnerability," said one former courtier.
"He has enormous soft power at his fingertips, but he is in a bubble and you become isolated and very dependent on the people around you to tell you the truth and what's actually going on.
"Of all the royal households the most courtly one is Clarence House, not particularly because Charles and Camilla encourage that, because they can't see it, but there are people around them who enjoy being courtiers as opposed to professional advisers, and they don't always like to give them the bad news, so there are people who are perhaps 'yes' people or sycophants and that is the danger of being in these kind of roles.
"William and Kate are much more grounded and rooted in the real world. That is one of Charles's main vulnerabilities, being surrounded by people who fly too close to the Sun sometimes.
"Camilla sometimes tolerates it because it makes her life easier but sometimes she will put her foot down."
Charles has not only become Head of State, he has also become head of the family, a role that is considerably more difficult for a brother than it is for a mother or father.
While Charles's relationship with William will be the most important in the monarchy - and one that will be relatively straightforward, given their alignment on so many issues - handling his siblings will be a different matter, particularly when it comes to the awkward issue of the Duke of York.
"What his mother managed to do is that she was the rallying figure, which is partly down to longevity," said one former member of the Royal household, a comment that is as true of his relationship with his family as it is of his relationship with his subjects. "It will be more difficult for Charles to do that because of the life he has had."
While the Princess Royal and the Earl of Wessex are unlikely to cause trouble for their brother, the Duke of York, who had lobbied the Queen for a partial return to Royal duties after his sexual abuse case with Virginia Roberts Giuffre was settled earlier this year, has a difficult relationship with Charles.
The Duke is unlikely to show the level of deference to Charles - who has had his own high-profile difficulties recently over cash payments to his charities - than he did to his mother, though Charles took an even firmer line with Andrew than the Queen did, and will surely leave him out in the cold rather than risking any taint to his reign.
Handling Prince Harry will require more subtlety. Even the Queen's death was not exempt from the all-too-familiar issues over Harry and Meghan, as the Duke of Sussex flew to Balmoral without his wife, prompting inevitable speculation that she was not welcome.
The King is likely to leave the door open for Harry and Meghan to take part in family events, if not royal ones, though it will be up to them to tone down their attacks on the monarchy, which will not be tolerated by either Charles or William.
When the Queen was crowned as a 27-year-old, her legacy would not have been at the forefront of her mind. For King Charles III, it will already be very much in his thoughts.
"Clearly the fact that he won't be there for that long will make a difference," said a former courtier. "Will he want to make some kind of big mark? It may be that stability and continuity will be the legacy he wants."
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