There's an interesting interview with Mr. Welsh in  the Huff Post UK. Below are the Filth portions plus a couple excerpts, but I think the whole interview is worth reading:

People like to say your work is about drugs, or violence or friendship – what do you think?
What I’m fascinated by is how people fuck up, the decisions that they make, the process by which people don’t act in their own interest. No matter how bad life is, we can always mess things up even more and find a way to self-sabotage. These mechanisms arise from a fear of failure, but also a fear of success. We live fairly short lives. The good things, like love and art, redeem our lives and make sense of them. So why people reject all that, and embrace the horrors of life instead, interests me as a writer.

How do approach the issue of morality in your books, particularly with the characters you are fond of?
I think you can show people being as bad as possible, but you have to show the consequences for the people involved and the society around them. You should never shy away from people doing bad thing, but neither should you shy away from showing the repercussions this has, and the forces that are acting on them to make them behave in that way. Otherwise it’s just about giving the reader a visceral shock. If we see what is driving them to do these things, then you’ve got proper human storytelling.

Which of your characters do you like most?
These four from Trainspotting, Bruce Robertson (Filth) and Juicy Terry (Glue) are my favourite characters. And Ray Lennox (Crime) too. They’re all interesting to me. It’s a pleasure spending time with them.

We’ve shot Filth now and that features one of the most incendiary individual performances I’ve ever seen from a British actor in James McAvoy, who play Bruce Robertson. That’s going to get massive attention I think.

Thanks, gen.  LOL Lionsgate (UK distributor) wants him to stop raving about James' performance.   I imagine the De Niro comparisons are making them squirm. Have to admit at first I had flashbacks to Mark Millar's overly enthusiastic comments/predictions about Wanted, but I don't get that vibe from Welsh. I think he's genuinely blown away by James. We understand that, don't we, McAvoyeurs?

Here are some interesting comments in this interview with Metro:

This week, the film adaptation of Ecstasy hits cinemas, while Filth, starring James McAvoy (‘better than Robert de Niro in Taxi Driver,’ says Welsh), is in post-production.

Sometimes, writing about people f***ing up f***s him up too. ‘All those embarrassments and humiliations come back,’ he says. ‘Filth [featuring Welsh’s most monstrous character, DS Bruce Robinson] and Crime [about a paedophile ring] were very difficult because you get into the character to some extent. With Bruce, I wasn’t right for about a year. I have certain rituals, though. I make myself do something normal for a bit and I’m back to being Irvine.’ Read more: http://www.metro.co.uk/lifestyle/896303-irvine-welsh-wanting-to-be-famous-is-the-most-revolting-thing-in-the-world#ixzz1skcKnLe0

Hmmm. Think James will have difficulty shedding 'Welsh's most monstrous character'?

Q & A with Ken Marshall on filming in Glasgow from the Glasgow Film Office

'Filth' Q & A - April 2012

Formed in 2006, Steel Mill Pictures is based in London and has produced several feature films over the years including the award-winning, critically-acclaimed 'London to Brighton'. Most recently the team came to Glasgow to shoot the Irvine Welsh novel 'Filth' about a bipolar, bigoted, junkie cop who manipulates and hallucinates his way through the festive period. Ken Marshall, producer on the film talks about his experience of filming the production here in Glasgow in 2012.

How long have you been involved in the production?
I started developing this project with writer/director Jon Baird approx 3 and half years ago.

Why did you choose to shoot in Glasgow?
The story is Scottish and primarily set in Edinburgh, so we felt we could make it work by shooting mainly out of Glasgow. Also, there is a very friendly, experienced and talented film community in Glasgow which made the decision quite easy!

What was the response from Glasgow Film Office / Glasgow City Council to your plans?
Glasgow Film Office (and in particular Jennifer Reynolds) were very supportive from the beginning; during our development phase and through principle photography. They were very easy to deal with and assisted us with logistics, contacts and a facilities grant

How many cast, crew, extras etc were involved?
Our cast was very big, close to 60 speaking parts. We hired about 80 Scottish crew and used many local companies.

How was your experience of Glasgow, would you recommend Glasgow for more shoots?
My experience in Glasgow was excellent; good crew, good people to work with. The city is very film friendly and surprisingly versatile for different settings. And Glasgow itself is a great city which I feel is really underrated.

More nice comments about James from Martin Compston in the Scottish Daily Record:

The Sweet Sixteen actor has also just finished filming the big-screen adaptation of Irvine Welsh's 'Filth' and he insists it was a nerve-wracking experience.

"It's my favourite book ever and I was nervous", he said. "I didn't see how they could adapt it but they've managed it.

"James McAvoy is the right boy at the right time to be leading it. He's at the top of his game and this is a cracking part for him.

"Being a short west-coaster, he's blazing a trail and I hope I can follow him. I play football with him and he's a great lad.

"You never stop learning and James' commitment was exciting to watch - I definitely soaked up things from him."

A bit more from Irvine Welsh in this interview w/ BBC Sport that's mostly about his obsession w/ the Scottish Cup final on Saturday.

Welsh, 53, will be in the south of France promoting films of his books Ecstasy and Filth, but he admits he will not be entirely focused on the Croisette schmoozing.

"I'm going to be at the Cannes Film Festival but I've cancelled all my engagements for the Saturday and Sunday," he told BBC Scotland.

"I'm flying to Edinburgh from Nice early on Saturday morning then I'll get back to the mundane task of selling my films, which I'm not really interested in right now, because this game is all I can think about.

"I didn't think Hibs and Hearts would ever make it to an East of Scotland Shield final again, never mind the Scottish Cup final! It's absolutely amazing."

Hmmm, wonder if James will be there...   At the game, I mean.