On TV, David Walliams is the ultimate class clown.
From dressing as a laydee on Little Britain to pulling his trousers down on Britain's Got Talent, he gives the impression of being a perpetually gooning Jack-In-The-Box.
But there is no such larking about at our photoshoot. I'm almost surprised when he fails to do a single prat-fall or camp impression. Hell, he doesn't even flirt with any of our crew - male or female.
So will the real David Walliams step forward: is he the show-off we are used to seeing on screen, or the quietly spoken man who turns up early to our shoot with a rack of clothes, shaking hands with everybody before getting on with the job without fuss?
After a couple of hours in his company, I decide he is both: able to lark about when the situation calls for it, and be serious and empathetic when it doesn't.
'I choose my moments on Britain's Got Talent,' he says. 'We record for 12 hours a day and you only see a bit of it, so it's not all gold. Sometimes I say something that isn't funny and doesn't get a laugh.
'Some of it's funny and some of it's crap. I try my best. And at other times it's just inappropriate to joke.
'We had a girl this year who was a victim of the Manchester bomb, and she did a dance routine, and because she is using a wheelchair, all her friends did too, so that was beautiful.
'I've been writing books for ten years and now I am always thinking of life in terms of stories. That was a beautiful ending to a story. Incredibly moving.'
The ability to straddle these two different sides to his personality so seamlessly seems to be key to David's success and longevity.
We may see him as an out-and-out comedian on TV, but the truth is that behind the scenes he is a serious (and highly successful) writer and businessman.
He shot to fame with Matt Lucas on their BBC show Little Britain: a larger than life, thoroughly OTT series of character sketches that were unashamedly populist. David and Matt were, for a time, the biggest comedians in the country, even playing arenas with characters like Vicky Pollard and Lou and Andy to a combined audience of one million fans.
When that finished in 2006, the pair continued working together briefly, but their next project - Come Fly With Me - was not as successful and they went their separate ways.
Walliams says the adjustment was not easy.
'It was quite hard,' he concedes. 'I remember – here's a bit of name-dropping – I said to Adele, when she was writing her third album, 'The problem you've got now is you're competing with yourself'.
'Because once you’ve written Someone Like You, people think, 'When is the next Someone Like You coming out?' but it’s actually very hard to sit down and write a sketch or a song and make the next big hit.
'We had this show that was successful beyond our wildest dreams, and then it was like, 'What next? I don’t know'. We did a series for HBO, we did Come Fly With Me, we did stuff, but it was hard.
'It was amazing being in a show that was so present in the culture. Every day you would pick up a newspaper and they would be referencing it in some way. But you can't sustain something like that. I likened it to being in a car and it's going downhill, and the brakes have been cut, and you're just trying to keep control of it. The juggernaut pinged to a halt. I'd never thought beyond that: being in a big comedy show was my life's ambition, so that was the end point.'
And yet, in a way, that was really just the beginning. Two years after the death of Little Britain, David wrote his first children's book, The Boy In The Dress, a touching portrait of a child who was different from his friends.
Although it wasn't initially a best-seller, the subject matter began to change the public's perception of him from a lightweight comedian to someone rather more thoughtful. In 2011 he swam 140 miles along the River Thames for Sport Relief, and his reputation took another major shift: the moment he rescued a dog who had fallen in the river in particular elevated him overnight to a national hero.
Three years later, he set up his own TV production firm, King Bert, with Miranda Hart and producer Jo Sergeant, which has already made 25 programmes, including well-performing versions of Walliams' own books.
Now, David is practically a member of the establishment: he was even awarded an OBE last year.
So does that mean he will only do 'safe' comedy now that he has such a reputation to protect? No more edginess? Little Britain and Come Fly With Me, after all, attracted much criticism for featuring characters that were black, transvestites and disabled, and Matt said in an interview last year that he would not do the sketches again, branding them 'a more cruel kind of comedy than I'd do now'.
'You'd definitely do it differently,' David agrees. 'Because it's a different time now. You'd make any comedy differently. We started working on Little Britain nearly 20 years ago because it was on radio first.
'It's hard to say specifically how it would be different. There's all kinds of tolerances that change. People understand people's predicaments more now. Maybe it's, 'We see this differently now, we've got more information about this now', and it would be a different type of joke.
'I wouldn’t rule out anything because I basically think you have to be able to make jokes about everything, everyone. Otherwise there is no point having comedy. But you have to obviously think about these subjects you deal with, and you have to think about the audience reaction, you have to think about whether someone won't like it.'
And yet some people were offended at the time. Did Matt and David think about that before going ahead?
'Yes, of course we did, we’re not stupid. Of course we thought about that at the time, but then also you’ve got to understand that comedy for me is celebrating things. There were gay guys at Pride dressed as Daffyd. So they have seen Daffyd as a funny character and they’re celebrating it.
'We went to a hospice for disabled children in Wales and the kids were in wheelchairs and they wanted to be like Andy with me as Lou, their carer. The idea for The Boy In The Dress actually came from a boy who sent me a picture of himself as Emily Howard, and now boys go to school in a dress holding a football and I feel I might have made it okay for some kids to be different.
'There's an assumption that someone on the 'receiving end' of the joke has no sense of humour about it, but we should have comedy about everything. There's nothing more funny that being sent up by your friends.
'I've got funny friends - Rob Brydon and Jimmy Carr - and they absolutely destroy me. And it's hilarious. You know?'
In the same interview, Matt Lucas admitted to apologising to Gary Barlow 'several times' after he dressed up as him in a skit for their sketch show Rock Stories (David played Howard Donald).
Is Matt, perhaps, more sensitive than David, perhaps more ashamed? It appears not as David says, 'I actually apologised to Gary Barlow as well.
'The thing is, we were doing this little show tucked away on a cable channel and we never thought we'd actually meet these people and we did go quite far with things. Then you meet them. I can't remember anyone complaining, though. Gary's a very sweet man. He's not the least bit bothered about him.'
I tell David that Gary has, coincidentally, been spotted in the studio next door to us, and he laughs.
'I heard he was! I'd like to say hello but I didn't want to break away and be all showbizzy and you'd write, 'He’s not doing the interview because he’s gone to do air kisses with Gary Barlow'.'
He's joking, but it's yet another sign of David's utter professionalism. When he arrived, in tracksuit bottoms and nursing a heavy cold, my heart sank. Celebrities who are not feeling very well are not, in my experience, the most fun interviewees. And there had been a few serious messages passed back and forth in the run-up to the interview between David's people and Radio Times, including a polite question to ask if the photographer would mind that David is currently sporting a beard.
But if I was worried, I needn't have been. He is open, honest, and thinks hard about what he wants to say.
A lot of our conversation is quite serious: he is very eloquent when I ask how he felt when he heard that Ant McPartlin - host of BGT - had been arrested for drink driving.
He says, 'Ant and Dec are popular beyond anybody’s imagination aren’t they? Every single person who comes to the show says, “Can we meet Ant and Dec?', and they are the most generous people with their time.
'And they are incredibly relatable and recognisable, and the hard thing about when you’re well known is that things can go kind of awry in your life, and it all gets played out in public.
'And I know, I’ve had similar things in my life. I remember when my father was dying and I was on the phone to him and people were coming up for selfless. It's hard because they want you to be smiley and happy and just like you were on the Jonathan Ross Show but you're going through something that's causing you pain.
'But I’m sure he’s getting some great help, and he’s genuinely lovely, what you see is what you get with them. Not everyone is the same as they are on screen, but those two are. I’ve actually known them for about 25 years and they've never changed.'
But during other parts of our interview David is quick to laugh - usually at himself - and great company.
We talk a lot about pinch-me moments David has felt starstruck - meetings over the years with Steve Coogan, Vic and Bob, Nick Cave, Roger Moore and Michael Caine, or the time he and Matt had to keep Kate Moss and Pete Doherty waiting because Sir Paul McCartney had popped into their dressing room to say 'hello'.
Then I ask about a down-to-earth moment where David felt his bubble had burst, and he thinks for a moment before saying, 'I really hope I can think of something better than, 'Well, I was having dinner with all the James Bonds, and the diamonds fell off my shoes…'.'
When I tell him he sold more books than JK Rowling last year he rolls his eyes and says, 'Yes, because she didn't have a book out. She's sold 500 million books so I don't think she's too worried'.
He is naturally funny and sharp witted. I suggest he may have put his comedy days largely behind him, he is mock offended: 'My books are quite funny, you know'.
And, in fact, his comedy days are not behind him at all. He currently has new comedy shows in development with the BBC and Sky - 'a studio sitcom, really hard to get right but that doesn't mean you can't try, and a multi-character comedy show, not a sketch show' - and is writing film scripts in America with Shaun of the Dead and Baby Driver writer Edgar Wright.
But most of his time is spent writing at home so that he can spend time with his four year old son Alfred, by his ex-wife Lara Stone. He credits the boy with his current happiness after undergoing bouts of depression and self-loathing in the past.
'All is good. My career is great, I've got a beautiful son. Of course there are things I worry about that sometimes keep me awake at night but I’ve got a lot to be very, very thankful for.
'Being a parent is a great thing because it makes you a lot less self-obsessed. All you really care about is the little person’s life and what the future holds for them. And your own worries really fade into the background, because you haven’t got time to think about that. All that matters to you is this person that you love more than you’ve loved anyone else ever before in your life.
'So, yes, my life revolves around him. Everything I do, I think about how it impacts upon him in some way, is it going to disrupt my time with him.
'It’s hard, because there’s lots of great things out there - I'd love to do more theatre - but I don’t want to do anything that’s going to take me that far away from him or stop me putting him to bed for six months.'
It sounds, I suggest, as if David is taking more and more of a back seat when it comes to performing. Will the future involve him retreating fully into writing rather than appearing on television?
He looks horrified at the thought. 'I mean - no - unless you've heard something I haven't?', he says. 'I mean, there’ll be an age where I’ll be doing shows about cathedrals. When you're slightly old you do find something you'd like to bore people about. 'Great Steam Journeys of Britain', perhaps, or 'Cakes! Tea Shops! Scones!'. How did the great British scone come about? Fascinating.
'I always used to wonder what I would do when I was 70 and I thought it would be great to be in Last Of the Summer Wine with Rob Brydon and Steve Coogan. I can't think of anything better.
'Sketch comedy might be a bit of a young man's game but I'd like to do more straight comedy. There's a day when you're pensioned off television unless you're Bruce Forsyth, but if people want me on screen I'll happily be on screen, I love it.'