I'm not so sure some of the contestants on TV's Britain's Got Talent, where Walliams has been a judge since 2012, would agree, although I am reliably informed by my daughters that's he much nicer than Simon Cowell.
Indeed, the father-of-one young son starts out polite and professional, becoming ever more generous and good humoured as it progresses. By its conclusion, we're swapping jokes and recommendations for classic children's books; right now, Walliams is a big fan of Emil and the Detectives.
The award-winning author probably doesn't even have to bother with interviews any more. He's sold about 13 million copies of his books which now number 10, including his latest release The Midnight Gang, and made millions from their sales; five of them have been adapted for screen. He's been dubbed "the fastest growing children's author in the UK" and "the new Roald Dahl".
He'd only have to send out a press release to ensure sales of The Midnight Gang strike it big. In September, he presented the UK's Channel 4 programme, Roald Dahl's Most Marvellous Book, which celebrated the late author's 10 bestselling titles.
So it's natural to start by asking how Walliams feels about the Dahl comparisons.
"Of course, it's enormously flattering but I think it's come about mainly because Sir Quentin Blake, who illustrated Roald Dahl's books, did the illustrations for my first two, so everyone went, 'oh, he's the next Roald Dahl'!
"There's also a slight comparison in that many authors for children write series, whereas Dahl didn't and neither do I; it's a different story each time. But the quality of his writing is so good and was so far ahead of its time, so it's not something I cling on to. Although it looks good on the cover of your next book.
"I really admire the timelessness to his writing; he wrote his books 50 years ago but kids still love them. I'm not sure there'd be many children's films from 50 years ago that kids would sit down and still appreciate.
"His work was completely unique; there were funny rhymes and great wordplays, fabulous characters, the likes of whom you wouldn't find in other books and his ideas were actually very complex. He was an utter, utter genius."
Dahl was the first author whose books a young Walliams (then David Williams - he changed his name to avoid confusion with another performer with the same moniker), growing up in Merton, London, fell in love with.
The son of a London Transport engineer and a lab technician, Walliams fondly remembers the more innocent days of childhood, when iPads hadn't been invented, video players were still in their infancy and an exciting weekly outing was a trip to the library. People communicated rather than connected, he says.
"You know, you'd go into the library and you'd be allowed to choose a few books so maybe you get one about space, because you're interested in space, and then maybe a cassette tape - a cassette tape! - of the soundtrack from Flash Gordon and then a book. I was lucky to choose Roald Dahl books because they hooked me into reading.
"I found them funny, a little bit scary and really entertaining. He wasn't trying to write or to be like anyone else; it's almost as if he created his own genre."
Walliams' characters are a bit like Dahl's; they're often the underdogs and the overlooked who are battling adversity in a world where the adults who are meant to be helping either don't or can't care. The granny, for example, in Gangsta Granny is a jewel thief while, in Grandpa's Great Escape, grandpa has Alzheimer's and his grandson is the only one who relates to his world of make-believe.
The quirky characters and scenarios allow Walliams to explore some of life's less pleasant aspects, as Dahl did, yet remain enormously entertaining. His first book, 2008's The Boy in the Dress, came about simply because Walliams wanted to try writing and to explore what would happen if a 12-year-old boy, searching for a female role model in the absence of his mother, wanted to wear dresses to school.
It developed into an exploration of wider gender issues: why are we told some hobbies and pursuits are more appropriate for boys than girls? Why do some boys - and girls - behave the way they do? Walliams says the best ideas grow from simple imaginings, bits of anecdotes friends relate to him. Take his second book, Mr Stink, about a malodorous man.
"A friend related this story from his childhood where every day, on the way to school, they drove past this elderly woman and one day, they decided to be kind, stop and offer her a lift which she very gratefully accepted.
"The trouble was she absolutely stank, so it was extremely uncomfortable for everyone else in the car. They never offered her a lift again and would feel terribly guilty because they were still driving past her on a daily basis."
It's amazing that Walliams has had the time to write as much as he has. When he's not acting, presenting or travelling - including to New Zealand - and promoting his books, he's raising millions for the charity Sport Relief, via endurance sports events.
He's swum the English Channel, the River Thames, the Straits of Gibraltar and cycled the length of the United Kingdom to raise money for vulnerable people in the UK and for developing countries.
Contracting giardiasis after the 2013 Thames swim, and injuring his back (and rescuing a dog), didn't put him off swimming. He says it's a great way to relax, stay fit and have uninterrupted time with his own thoughts. It's during his regular swims that ideas start to take shape.
He works most often with illustrator Tony Ross, who he says he must surely annoy.
"I think illustrations are a very important part of children's books, especially in a more visual age, so Tony gets a brief to do 50 and I come along and say, 'no, I want 200! There must be a picture on every page!'"
Walliams agrees that reading is important and that most of us, adults especially, live lives that mean we are far too distracted to do enough of it. But he's adamant that a love of reading stems from childhood and that youngsters can't - and shouldn't - be bullied it into reading at school. That's the quickest way to put them off. It comes down to finding an author whose writing you love, as he did with Dahl.
He wrote his last book, The World's Worst Children, specifically to try to engage with the child - usually a boy - who sits at the back of the hall where Walliams is speaking and says he doesn't like reading because books are boring.
"It's about hooking them in and not making reading seem like a chore," he says. "I think reading is important because not only do you miss out on great literature if you don't do it, but also you miss out of finding out about new ideas and the opportunity to use your own imagination.
"As a child, I think I was most interested in - although I wouldn't have put it like this - the feelings of powerlessness that you have because your grown-ups make most of the decision for you so you like to look ahead and think about what it will be like when you can figure things out for yourself.
"I actually liked the idea that there might be consequences for your actions; that the world was a little bit magic. That if there was a curtain twitching, then it might be a ghost. That's what I got from Roald Dahl."
The Midnight Gang by David Walliams
(HarperCollins, $25; hardback $40)
