Was part of the attraction to this project the fact that it's so far removed from The Wire?

The thing that attracted me the most was that I was playing Cromwell, who is obviously a contentious character and I think playing historical figures is always fascinating.

It sounds like you already knew a fair bit about Cromwell.

I knew a bit about Cromwell from school, and then some prejudice from being from an Irish Catholic background. My mum is Irish, and still isn't speaking to me. My wife-to-be is Irish. Believe me, he's not a popular figure over there.

Did your opinion of him change as you were playing him? Did you discover a more sympathetic side to him?

I read a lot about him, and I came to admire him hugely - even though my opinion of what he did in Ireland never changed. He was the bastard of popular repute, there's no question about that. The Irish were just seen as beneath contempt - that wasn't just Cromwell's prejudice, it was a feature of the time.

He was an extraordinarily impressive figure, as a politician, thinker and soldier, wasn't he?

Remarkable. The thing that drew me to him initially was that he didn't do anything before he was 40, which I found very comforting, coming up to 39. And the next thing that appealed to me about him was that he cancelled Christmas, which, when you think about it, is a really good idea. Antonia Fraser's book about him is called Our Chief of Men, and at the time he was unquestionably the greatest leader in the world. Had he lived another ten years, we'd have invaded Spain and France and Holland, and the British Empire would've happened 100 years earlier. He was an absolute Titan.

In the BBC's poll for the greatest ever Briton, he was in the top ten. Does his exalted status add a sense of responsibility when you're portraying him?

But what's nice about him is no actor has definitively played him. Loads of people have done him, but when you think of him, you don't immediately think of one actor, the way you might do with Elizabeth I. I suppose I wanted to just catch a fraction of his personality.

All the armour, and riding about on horses and so on - is it good fun, or really difficult?

It's really hard, but really fun. I'd always thought the Roundheads' stuff was a bit uninspiring, but when you actually put the kit on, all that leather and armour and thigh-length boots, it was just so sexy. You think the Cavaliers have a better look, but they don't - they look quite poncey. But in terms of clanking around in all that armour, I often felt I was in Carry on Cromwell. Monty Python's spectre is never too far away in these things, either.

As with 300, this saw you and Michael Fassbender playing soldiers together again. It's a boyhood dream, dressing up as soldiers?

I had more fun on this than anything I've ever done. Mainly because of Michael and John Simm. By the end, we were saying we didn't ever want to do any role again that wasn't on a horse. Sword fights, charging on horseback, it was brilliant, exactly what you always wanted to do from the age of six.