David Walliams explains how Grandpa's Great Escape is a story about the special connection between a grandson and a grandfather.

What was the inspiration behind Grandpa's Great Escape?

I heard about a real story a few years ago about a man called Bernard who escaped from an old peoples' home so he could re-join his soldier comrades in a reunion. He has since passed away, but I found myself thinking, 'I wonder if there could be a Great Escape type version of that story?' because I love that movie.

Also, I was presenting Comic Relief and I kept seeing this film about an old lady who was living with Alzheimer's, and she kept talking about how she thought her husband was alive, when it reality he had passed several years prior. I saw that film about 10 times and it made me cry every single time. So I had these two ideas that I was trying to piece into one, and I wasn't sure if I'd be able to mould them together as one story is quite comic, but the other is ultimately very sad. But I think it strikes a good balance.

This is your 5th children's book to be adapted for a Christmas special. Do you write your books with potential TV adaptations in mind?

I do tend to think visually - so, I'd say so. But they do need to work as books in the first instance, so there's a fair bit of work that goes into adapting the story. Given that I come from writing television, it is often at the back of his mind. I think about the characters and how they could be acted out when I write it.

My main objective is to make the story very exciting and visual - I'm trying to make them as entertaining as possible for children. I want to encourage kids to read and give them something exciting and funny and moving. And I try and make sure the stories have a lot of pace, which means they adapt a bit more naturally for TV. But this story is quite ambitious - it's about the stealing of a Spitfire from a war museum! This is quite difficult to do on television given that we don't have the multi-million dollar budget of a movie! But there will be a lot of special effects in this one, and I'm really excited to see how it all comes together.

How involved are you in adapting the books into screenplays?

I work with a great comedy writer called Kevin Cecil. We've been working together for a few years on my book adaptations, and have worked together on other projects too. And he tends to do the first draft and then I come in later and we start working together on it. Because he's adapting my book, I tell him not to be slavish to it - he can change things and bring in new characters if they are needed. But Kevin does the first draft, which I personally think is the hardest part - that's the real heavy-lifting when you're writing a script. And then I come in and co-write the subsequent drafts. It's pretty harmonious.

You've worked with Jennifer Saunders on a number of occasions. Do your comedic and acting styles complement one another?

I'm not sure - I just see her as a hero! She's been involved in some of my favourite comedies of the past 30 years. Right from The Young Ones, obviously through to French And Saunders and Absolutely Fabulous. She's been in amazing things that I've loved, so I'm always a bit star-struck when I meet her. I can't help but think "Oh my God, that's Jennifer Saunders!"

I'm really chuffed that she's in Grandpa's Great Escape, and she is brilliant in it. And I do think we complement each other. We're not a millions miles away from one another in terms of what we've done - comedy and sketch characters and spoofs. But I do hold her in very high regard; I wouldn't put myself in the same league. She's a superstar.

Sir Tom Courtenay is clearly an esteemed, respected actor. How did you feel when he agreed to be part of the cast?

I was delighted, because we really did need a heavyweight actor like him. As actors get older, it's only really the great ones who are still doing it. Unfortunately, it becomes quite a short list. And they're all legends and massive movie stars.

Tom was always my first choice for the part of Grandpa - I met him a while ago, and started corresponding with him and we now have a running joke that I'm his only fan. I'd been raving about some of his previous work, and then when we finally met again he said, "ah, it's my fan!" and I replied, "yes, and it's very lonely being as I'm the only one!"

But we've been chatting for a long time, and sending each other pictures of our dogs. I've loved him forever, and it's very special when you meet someone who's a bit of a hero of yours and it turns out they want to get to know you too.

When he was offered the part, he called me because he had a few concerns - who would play his grandson, for example. So we've been working together very collaboratively. It's been really good fun, and we leave each other little presents in one another's dressing rooms. I'll leave him some chocolates and he'll leave me a book of poetry - because it turns out I'm a feeder and he wants me to be a little bit more intellectual!

I love watching him back, because his performance is poetic - and this part really needed that, as Grandpa lives somewhat in a fantasy world. We've all been a bit in awe, and I can't take my eyes off him when I'm acting opposite him. If he's not nominated for a BAFTA for this, there's no justice because his performance is a thing of beauty!

How would you sum up Grandpa's Great Escape in a few sentences?

It's an adventure story about a daring escape from an old people's home, and it's also a story about a special connection between a grandson and a grandfather - because Jack is the only one who understands Grandpa. I often feel there's a special connection that skips a generation.

What do you hope audiences will take away from the show?

I hope that people might be more sympathetic to those with Alzherimer's or similar conditions. I hope they might think a little more about the incredible sacrifice people made during the Second World War. I grew up somewhat under the spectre of it - I was born in 1971, but my parents both lived through it and my dad can certainly remember it and I think we need to continually learn from history.

And overall, I hope they're entertained, I hope they're moved and I hope they ultimately treasure their grandparents a little bit more, because I think time spent with your grandparents is a very precious time. I can remember very special times spent with my grandparents, which I will always treasure.