The Welcome to the Punch star talks about playing the bad guy in director Eran Creevy's new cops-and-robbers thriller - and a lot more besides.

What was it about Welcome to the Punch that appealed to you?
Mark Strong:
What I like to try and do are things that are as different as possible to what I've already done, and I've never really done an action movie – not like this, a cops-and-robbers crime thriller. So that was fascinating when it came along. I was surprised that I had to go and get fit for it because I thought I was old enough now to have avoided all of that stuff! But I had to run up and down all day and all night, jumping over things etc, so that meant I had to be fit.

The script really appealed, though. I remember reading it and wanting to find out what happened at the end. What was also interesting was that under the guise of an action movie, with guns and car chases and all of that, there's also this story about guns themselves – about the idea of arming the police and the idea that politicians might be using the whole issue of gun crime to get votes. I thought that was quite clever. Within a film that purports to be a brainless action movie, if you will, there's actually a very cerebral plotline going on about guns themselves.

Speaking of getting fit, is that all you in your shirtless scene or was there any CGI?
[Laughs] No, but I wish they had done that. But the thing about it is that I didn't have to be big; I just had to be fit, and that just meant stopping drinking, going running and going to the gym. I quite enjoyed that, I have to say, because at the same time I was told I had high cholesterol and a heart murmur. Just before we started filming I had some reshoots on another project, I had a medical and the lady said: 'Did you know you had a heart murmur?' To get that checked out I also had to have a blood test and they went: 'Did you also know you have high cholesterol?'

That's the thing about being nearly 50, isn't it? You get to a certain point where you have to think about all that stuff and it coincided with doing this film. I've kept up the regime, actually, and it's been a revelation; I'm fitter now than I've ever been in my life.

What do you think about the police in the UK being unarmed? Should they be armed?
Absolutely not. I can't speak for the police - I don't know what it's like to be on the beat and I don't know what their attitudes and opinions are - but personally it seems to me that wherever there are armed police it attracts more armed robbery or armed crime because the criminals take guns with them in case they're apprehended. I don't know if it would change if the police over here were armed but I suspect there'd be a lot more gun crime. I'm quite proud of our country for not having armed police.

After all that charging around in Welcome to the Punch, is it a relief to get into the studio and do a quiet Who Do You Think You Are? narration?
I love it that in my film life I'm a baddie but in my voiceover life I'm a goodie. I'm an informative, kind, good person. I also do the voiceover for Give Blood, the blood campaign, and it's always struck me as ironic that in my film life I'm a villain but with voiceover I'm a good guy.

Have they ever asked you to be the subject of Who Do You Think You Are? yourself?
No, they haven't. My wife's desperate for them to do so because she'd love the kids to know. I don't really know my history much beyond my parents because my dad wasn't really around and my mum's family are all from Austria – so there is stuff to be discovered.

And who would do the narration for your episode?
That's the problem, isn't it? I can't really surprise myself!

You mentioned how you often play villains. Do you enjoy getting to explore that?
Definitely. I think that's why I like, and don't mind playing, villains because it's acting. It's so far removed from me that I can see it as a discipline. I think what must be really hard - and I haven't done it very often - is to just play me – to just be natural. Those leading men who play themselves, it's a very difficult thing to do. I come from the theatre, which is all about transformation, and I've been lucky enough to have a film career that's also about transformation. I can play aliens, Arabs, villains, spies and there's a really lovely mix to all of those.

So reality TV is a no-no?
I wouldn't be able to cope with it!

You recently said on the radio about being the 'but' in movies. Can you explain that further?
I was on a show with Mark Kermode and Simon Mayo on [BBC] Radio Five. The week before Tom Wilkinson was on and the subject of billing came up – how when you've got a lot of stars in a film who gets first billing? He mentioned that there's the 'and' and the 'with' so you get first billing, second billing, third billing and so on. Then you get 'And Helen Mirren, with Tom Wilkinson', so somehow it looks like you're special. So I said: 'What about the 'but'?' Can you imagine a movie where they said: 'And so-and-so, with so-and-so, but also'?! Insert name of actor you don't like!

Welcome to the Punch is in cinemas from Friday March 15.