Can you give a brief synopsis of Filth?
It's about a cop in Edinburgh who appears to be in control of his life. He is manipulating everyone around him to get a promotion at work because he thinks it'll help him get his wife and daughter, who have previously left him, back. At first we think he is in control and then we realise he is slowly falling apart. The tragic and hilarious events unfold as we go through the next 97 minutes. It's a tragic love story really.

Was it difficult to raise money for the film adaptation of a book that seems almost unfilmable due to how grotesque it is?
It was very difficult, and so we had to finance it out of four or five different countries and shoot in those countries too. It was extremely tough. People tried to adapt it before but were unable to get the right tone.

Did you think that the only way to do it was to make it the surreal, black comedy that it's turned out to be?
I think that was the only way to let people into it, to make it accessible through humour. That was the key, getting them to laugh first, and if they laugh in the first five minutes then you've got them and if not, you don't. Thankfully a lot of people seemed to respond to it.

How was working with James McAvoy? He seems to fully and totally commit to the character.
That's exactly what he did. He was needed every day, he immersed himself in it, and to get a performance like that you need to and James did. It's one of the great movie performances. He did an amazing job. I don't think anyone would've been more committed than James.

Do you think he was also key in bringing a level of humanity to what is a pretty repulsive character?
He was indeed. The knowledge of who James McAvoy is probably helped the character initially, getting audiences into the cinema. But then I think people would have been surprised at how far he actually went with the character also. So I think it ended up working on both levels.

Were you quite worried about having a film with such a dark central character?
No, because people do love an anti-hero, whether it's A Clockwork Orange, American Psycho, In Bruges, Bad Santa or Bad Lieutenant. People are fascinated by interesting, dark, complex characters. There are darker films than Filth. It's a black comedy, so there's a certain kind of person who'll get it and a kind of person that won't. You can't please everybody, so you have to make the kind of film that you'd want to see.

Were you in discussions with Irvine Welsh himself while you were adapting it?
Irvine never had any input into the script. I sent it to him and he told me not to change a word. He was very supportive from day one, which thankfully has helped me a lot. When he saw the film, the first thing he said to me was that it was better than the book, which was a very humbling thing to hear. He's a great fan of it, a great guy and a close pal. He's been a rock for me in this process.

It seems to have been a very good year for Scottish cinema (with, albeit a very different film, Sunshine on Leith as well as Filth), why do you think that is? Is it just a coincidence?
I hope it isn't a coincidence. I hope this is a start of something new. We can't sit on our laurels. We have to support the industry but we have to let people make different films, like Filth and Sunshine on Leith, that are commercial films. Because there are a lot of films made in Scotland that people never go and see. That's a lot to do with the filmmakers too, who should take responsibility as well. If they're given money to make a film, they should make a film that will find an audience, there's a duty there. So I really hope it's not a flash in the pan.

What are you working on next?
I'm working with Danny Boyle now on (TV drama) Babylon for Channel 4, shooting the first 3 episodes, the pilot of which airs on Sunday 9 Feb.

What can you say about Babylon, has that been an enjoyable experience working with Danny Boyle?
Yeah he's just brilliant; I did the job because Danny was involved when he called me up to direct it – a great phone call to receive! It's written by the guys that did Peep Show and seems to be a lot of fun.

What can viewers expect from it? It looks like something in between drama and comedy, certainly something far more dramatic than what the Peep Show guys have done before.
I think it's definitely more of a drama with comedy rather than a comedy-drama. People can expect a different look at the police, more through characters than the organisation itself. It is fun and there are some great performances, and looking at the scripts for the series there's a lot of big, juicy things coming up.

Filth is out on DVD from Mon 10 Feb. Babylon is on Channel 4, Sun 9 Feb.

Filth - Official Full Trailer