"But their success and the approbation is a measure of just how incredible these films are and what high esteem they're held in. The sheer scale of this undertaking, the numbers involved and what they've achieved are truly staggering," he adds, seriously.
"Also, you have to remember it was never guaranteed that they would be successful or that each new instalment would be bigger than the last one because, in the beginning, Peter was a bit of an unknown quantity: no one knew who he was, or what he was capable of.
"Actually, when I was first approached to do these films, I really wasn't that interested because I couldn't understand how a guy, who made splatter films like Bad Taste or Braindead, could do justice to The Lord of the Rings or why he wanted to film it in New Zealand, of all places. I thought it was preposterous.
"How wrong I was though, because when I met Peter I was so impressed by his passion and his vision and now I can see that there was no one else - and nowhere else - to make these films. I'm just so glad that I decided to give it a go, because it's really been the most remarkable project I've ever been a part of. It truly has been a life-changing experience."
A veteran of the British stage, McKellen's screen roles were largely confined to modest UK films before he became a major figure in two franchises - not just Middle-earth but as Magneto in the X-Men series. It could have been more - he was asked to replace the late Richard Harris as Dumbledore in the Harry Potter films, but refused, saying playing one wizard was enough. And his Gandalf had a fair amount of screen time as well as getting most of the franchise's best lines.
Orlando Bloom, Peter Jackson, Martin Freeman, Sir Ian McKellen and Evangeline Lilly at the World premiere of the film The Hobbit: The Battle of the Five Armies in London. Photo / AP
So a fair chunk of McKellen's past 15 years has been spent on the other side of the world from his London home, running around the landscape wearing a pointy hat.
"One of the things I loved about this film is because we had these wonderful locations, if you needed to film a scene on a mountain, then you'd go to a mountain and film the scene. So, when you see me on top of a mountain and I whistle and Shadowfax comes over the brow of the hill and trots up to me, then yes he really did. That was me, that was the horse.
"Likewise, I've been to the Forest of Fangorn and I've been to Gondolin, so much of it is real.
"Similarly, when there were the battle scenes in this final film ... okay there weren't 10,000 soldiers but there were 500 extras, which does help you to imagine it all. Also, when you see me on top of the battlements, wielding my heavy sword, I was really doing that."
Like McKellen, Orlando Bloom has been there from the very beginning of the quest right to the end, as the elf, Legolas.
"It's been about as life-changing as it can possibly get doing these films, because I've spent four years of my life in New Zealand, from when I was just 21," he says, smiling happily. "Legolas was the first role I ever got to play and it's also a character that has resonated with people the most in my career, so it's definitely defined me.
"That's why, when Peter asked me to come back to do The Hobbit films, I jumped at the chance.
"It was a joy to come back because it meant I got to reconnect with New Zealand, which was a really big part of my life, playing a character that had been a huge part of my life too."
The filming of the final instalment of The Hobbit trilogy was a bittersweet experience for Bloom, especially as the end loomed.
"The last day of shooting was very emotional for me. It felt like that was it, that it was the end of a long, but incredible, journey," he says, sighing involuntarily.
Read more:
• Dominic Corry: The wild world of The Hobbit
• The Hobbit: On set as the Battle of Five Armies unfolds
"Pete had cued-up some really stirring music for my last shot of the day, which was one of the final battle sequences, so it was all very dramatic standing there, waving my sword above my head. It's a moment that I'll never forget.
"But although I'm sad to see it go, because it's been such a huge part of my life, there's also a natural feeling of relief that this huge thing has finally come to an end," he says.
"It means I can now look forward to doing something else, playing different characters and starting some new chapters in my life. Thanks to these films, I have the opportunity to do that."
For Martin Freeman, who plays Bilbo Baggins - the star of The Hobbit trilogy - filming the final scenes of The Battle of the Five Armies proved to be an emotional roller-coaster ride too.
"Although I hadn't been a part of the Lord of the Rings trilogy, I'd spent two-and-a-half years with these people, on this job, and it has been a huge part of my life. So, it ended up being quite emotional, when I realised it was over - a lot more emotional than I thought it would be," he admits. "Actually, at first, as the finishing line was in sight, I felt quite happy it was nearly over because I was mentally and physically drained by the end of the film, so wanted it to be over, to leave it behind and to move on.
"But then something happened when I was filming one of the last few scenes with Graham McTavish, who plays Dwalin," recalls Freeman. "When we finished filming he came over to say goodbye and his words caught in his throat, from all the emotion [he was feeling], and I suddenly thought; 'Christ, I'm going to go here, too' and my eyes started to well up. I think it's because the enormity of what I was doing - and that this really was the end - suddenly hit me. I wasn't expecting that. Normally I'm quite good with goodbyes."
Who: Ian McKellen, Orlando Bloom and Martin Freeman
What: The Hobbit: The Battle of the Five Armies
When: Opens at cinemas on December 11
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