After a spell in the wilderness, reformed party boy Colin Farrell does his best on-screen work for ages with the perfectly pitched black comedy In Bruges.

There's no doubt that Colin Farrell is the proverbial colourful character. Dreaming of following in the footsteps of his father and grandfather in playing professional soccer, Farrell gave up on that ambition when, at age fifteen, he decided that he "never wanted it enough". Despite a dubious stint modelling underwear and a failed audition for boy band Boy Zone, Farrell's rise through the acting ranks was almost instantaneous. The double shot of Joel Schumacher's Tigerland and Phone Booth promoted him swiftly to Hollywood headliner status.

Soon though, Farrell's hard-partying Lothario ways started to overshadow his work. Top of the list was an internet-ready sex tape where a shaven-headed Farrell (he was filming Daredevil at the time) manages to get "breakfast, lunch and dinner" all in one helping. Such behaviour soon took its toll, and after filming Michael Mann's Miami Vice, a burnt out Farrell suddenly went AWOL. The late Heath Ledger actually had to step into his shoes by taking over Farrell's role in Todd Haynes' I'm Not There when the actor couldn't be contacted.

While Farrell's hoped-for return to form in Woody Allen's awful Cassandra's Dream was a major disappointment, the smart black comedy In Bruges more than makes up for it. In Irish playwright Martin McDonagh's feature film debut, Farrell plays Ray, a novice assassin killing time with fellow hitman Ken (Brendan Gleeson) in the picturesque Belgian town of Bruges. When the duo finds out that their manic boss (Ralph Fiennes) is coming to town, they soon realise that Bruges' odd mix of beautiful drug dealers, ketamine-taking dwarfs and American tourists are the least of their worries.

Do you ponder on the issues of guilt and redemption that are very much at the heart of In Bruges?
"I try not to, but I'm well aware of their existence in most people on some level. We've all done things. Only by taking stock of past transgressions can you move on from them. If you don't really deal with things, whether it's in the moment or retrospectively, then they will possibly - like a cancer somewhere inside - hold you back. But I don't mull over guilt and shame and all of that. I'm not a practising Catholic anymore!"

When did you stop being a practising Catholic?
"When I was about seven! I decided to be an altar boy, but I only made it through one mass. After that, I decided that school was actually better. You couldn't slag the mass when you were on the altar. But going back to your original question, no matter how heinous a particular crime perpetuated against somebody else is, maybe the only possibility for forgiveness or redemption is if you're able to forgive yourself. That, of course, is an incredibly hard thing to do, and that's what my character struggles with in the film."

Do you feel that you have a different lifestyle now, as opposed to a few years ago?
"What was my lifestyle like before? I don't know, man. Yeah, it is a little different, I suppose. It's a little bit calmer than it was before. I'm 32 now, and I'm a dad as well. I was at the party for long enough, and then the party starts to get boring. You start to hear yourself say the same shite every single night to different ears pretending that they care."

A bit like these interviews.
"[Laughing] Yeah! With journalists who pretended that they cared as well! I just seemed to make what seemed to be very natural changes. There's no major thought of, 'What am I going to be now? How do I want people to perceive me?' It's nothing to do with that. You can't help people's perceptions, you know? You can just act on who you want to perceive yourself as."

You stopped working for a period of time after Miami Vice; was there any particular reason?
"Ah, yeah, I was bollocksed. I was knackered and worn out. I'd been burning the candle at both ends, and it met in the middle eventually - there was no wick left! So I regrew my wick [laughing] and went away and just took stock. Now that I look at it retrospectively, it looks like one of those seven year chapters that people talk about. I'd only had six months off either side of when I did Tigerland to when I did Miami Vice, and it just seemed to be a natural time to step away."

In that period, did you spend most of the time with your son or your family? What did you do?
"I went to Bruges to do a film with Martin McDonagh."

And before you shot the film?
"I read the script and loved it."

And before you read the script?
"I heard it was coming, and it came."

In Bruges is released on September 4.