David Walliams talks about his one-off BBC One special.

What was the inspiration for Mr Stink?

Well I suppose the starting point was 'how much could you love somebody that really really stank?' A friend of mine told me a story about how they used to see an old lady on their way to school, and her mum stopped to give her a lift and she really smelt, so they never gave her a lift again. That got me thinking...

Had you intended for Mr Stink to be adapted for TV?

I had only hoped. The good thing is when you're adapting your own book is that you can make improvements. One of the things I did was to create more suspense. There's a really big element we have in the TV version that we didn't have in the book is that Mr Stink's smell is something that you can actually see. It's a special effect and it has powers of its own. It's almost a character in itself and that's one of the main reasons the show will be in 3D.

Did you enjoy adapting Mr Stink with Simon Nye?

I hadn't worked with Simon before. He is a writer I really admire and of course he has a long list of brilliant comedy credits, most famously Men Behaving Badly. He made a load of brilliant brilliant changes and brought in a lot of humour that I had missed. So I was delighted.

Were you tempted to play the role of Mr Stink yourself?

No because I don't think I'm a good enough actor. Hugh Bonneville is one of the nation's favourite actors, especially because of the success of Downton, and he has proved himself to be as equally brilliant at drama as he is at comedy. There's not many actors you can say that about. Twenty Twelve showed he was a master of comedy and most of the rest of his work shows he's a great dramatic actor, so we were really lucky to get him. I'm sure he's the reason many people will watch. Downton Abbey is bigger than X Factor.

Did you enjoy playing the Prime Minister?

When Quentin Blake did the illustrations for the book he told me he made the PM have the very worst aspects of Tony Blair and David Cameron wrapped up together. He's a Prime Minister who is much stronger on presentation than content and he wants to use Mr Stink as a photo opportunity to make himself look good. Obviously the real Prime Minister would never do anything like that! I had a bit of fun. I'm not doing an impression of David Cameron, because I can't do impressions anyway and I don't want to make it overtly political because I feel all politicians are ultimately as bad as each other.

Tell us about the rest of the cast please...

I know both Sheridan Smith and Johnny Vegas a bit and they are both highly skilled comic actors. And Sheridan has proved herself recently to be a great dramatic actress as well. She's the girl of the moment. And Johnny's background is as a comedian and he's really really funny. But also he's got one of those faces where you really feel for him and you really love him.

I'm delighted with the children who play Chloe and Annabelle - Nell Tiger Free and Isabella Blake-Thomas. Nell who plays Chloe has one of those faces that when you see her on screen you really care for her. She's got that quality on screen that you can't bottle. Ultimately a lot of the emotional impact of the story rests on her shoulders. She's got to be brilliant and fortunately she is.

Isabella proved herself to be really fun in Just William and Annabelle is really a comic part so she is perfect for that.

Raj is in all my books and I always wanted Harish Patel to play him. I knew him from Run Fatboy Run, which I was in briefly, and he was hysterically funny. I was actually really star struck meeting him as he's such a gem.

What made you cast Pudsey from Britain's Got Talent as Duchess?

I was instrumental in getting Pudsey. I've worked before with animals and so often you're told 'don't worry the dog's definitely going to do this' and it never does. So I saw Ashleigh and Pudsey do three routines in Britain's Got Talent and the dog never put a paw wrong. And he's really lovable - a lot of people will tune in to see Pudsey. Pudsey's the most famous dog in England so he's the dog we want in our film!

What will you be watching at Christmas?

What I look forward to are the treats that aren't on at any other time of year. You often get some real gems like The Borrowers. While the book Mr Stink was aimed at children the TV version is very much aimed at families. The great thing about Christmas TV is that you tend to watch it all together and so it's got to be something that has a broad appeal.

What are you expectations for the TV version of Mr Stink?

Well I hope it will make you laugh and make you cry. There is an emotional aspect to the story, especially in the hands of these brilliant actors. At the read-through I had tears in my eyes listening to Hugh read the scripts. So hopefully it will have that effect and I think at Christmas you feel like you want to watch something which has emotional impact, don't you?

The message of Mr Stink is to treat other people with kindness, whoever they are. It's quite metaphorical and it's a good one for Christmas. At Christmas if you're lucky enough to be surrounded by your family, you are made aware about those people who don't have that for whatever reason. Personally I can't help feeling a sense of guilt for having a fortunate life and also a desire to help other people. So I think that's the message of the book.