The scene on stage is tense. It's the night of 5 July, sometime in the 1970s, and in a New York precinct, four men are in an interrogation room. Two are cops, Kelly and Delasante, and two are suspected murderers – one a punk kid called Jimmy "Rosehips" Rosario, the other Sean, a bearded, middle-aged teacher. The latter are wanted for pumping old Mrs Linowitz full of lead; the former are not averse to beating seven shades of ordure out of them to get a confession.
The opening of A Prayer for My Daughter, revived by the Young Vic, is classic police procedural stuff, all wisecracks, threats, verbal fencing and people saying "son of a bitch". But gradually the dialogue changes, and in Act II, when Sean recalls his pietà-like encounter with a dead soldier, over whom he wept for 12 hours, we hit an extraordinary moment. Sean tells the cop: "There's a woman inside me, officer, and she aches for the men she has known."
It's electrifying, not just for the line's bittersweet melancholy, but because it issues from the lips of Sean Chapman, an English actor who's long been a byword in tough-bastard masculinity.
The stage crackles with unspoken desires and impulses as he and Corey Johnson (playing Delasante) fence and parry, and the burly Brooklyn-voiced toughnuts struggle with a vulnerability they've never confronted before. By the end, each of the four men will have spoken about a daughter – real or imaginary – with whom he has a transcendent, healing relationship.
It's a remarkable drama, written in 1977 by Thomas Babe, who was obsessed with American notions of heroism and masculinity and the uncertain emotions that lie behind them. He wrote about strained family relationships and the ways right and wrong are perceived. But didn't his characters come across as just a little peculiar?
"The Greenwich Village audience in 1977, for whom Babe was writing," Chapman says, "would have had a working knowledge of all the people on-stage, and the context of their lives. It was a 'Nam world, a world in flux, where things changed quickly, where the police were as bent as the people on the street. His play catches the essence of people wondering, 'Where's the morality gone? What's the code now?' So when you're preparing the character, you're always looking for the back-story."
So how do we take the character of Sean, the teacher and manipulator of street kids? Do we assume he's gay, bisexual, a furtive transsexual? For Chapman, the question is crucial. "I think what Babe is saying is: until we go beyond all these labels, and stop seeing each other in such terms, we're screwed. The cops at the start see the two suspects as gay junkies, and it gives them permission to deal with them as they like. Babe is saying that kind of morality is redundant.
"It's a play about a masculinity I recognise today. My experience of men, when I'm with actors and writers, is that there's an inner emotional life we all share. When I'm with men on the street, all that's invisible. If you're not talking about Chelsea at Stamford Bridge, or about the latest figures in the City, you're not talking about anything."
So what will save us men? "The heart of the play is, I think, about finding your feminine side. Of course, that's just another label. What we mean by a 'feminine' side is a creative, sensitive, alert, general awareness of the world. We've delegated the responsibility of sensitive reactions and intuitions to the women and said, 'Sorry, no, we men don't do that.' Babe is saying that, unless we embrace these intuitions, we're only half alive."
It's extraordinary to find such a soft, thoughtful, London delivery issuing from the large, truculent longshoreman you've just seen in rehearsal. Chapman is 46, and has been a professional actor since he was a teenager. His career has been crazily eclectic: from appearing in tiny provincial theatres in Yorkshire to the National, where he starred in the epic Angels in America in 1991, to playing dangerous-looking heavies on TV in Murphy's Law and Silent Witness. He hasn't set foot on stage since 2006, when he was in David Hare's adaptation of Gorky's Enemies at the Almeida; but he recently appeared with Angelina Jolie in last summer's A Mighty Heart, playing an American journalist.
The ups and downs of acting work is a leitmotif in his conversation, and has prompted him to write a cautionary tale. His first novel, A Distant Prospect, currently being offered around British publishers by Ed Victor, tells the story of Riley, a talented actor who struggles to keep solvent and feed his costume-designer girlfriend and her baby.
It is, you won't be amazed to learn, based to a large degree on Chapman's life. "To survive as an actor when you're young, you need massive amounts of adrenalin and commitment and you make the best of everything," he says. "Then you become less disposed to take everything you're offered. I've spent some miserable times doing rep in regional theatres. You find yourself in Huddersfield on a wet Wednesday afternoon, you're in a bad play with people you don't get on with, you're 25... That is pretty wrist-slashing stuff."
He pauses. "Now, after a gig like the Young Vic, I might not be in work for four months, then someone will ring and say, 'Look, there's an episode of Heartbeat...' and you have to make a quantitative decision about whether there's enough money in the bank for you to be able to turn it down.
Wasn't there a qualitative decision to make as well? Like, say, doing Heartbeat, but not The Bill? "The real question is: if I'm not going to enjoy myself, what's the point? When you're on the journey out, in your twenties and thirties, when you're married with young children, you'll do anything. But part of the reason I wrote the book is that I want young actors, starting out now, to see that qualitative judgements aren't possible any longer."
Chapman's career started with a bullet at 17, when he landed a role in the film Scum. He was born in Greenwich, London, in 1961 but his childhood was blighted by parental absence ("My father arrived at the hospital where I was born, put down his suitcase and said to my mother, 'I told you it was the baby or me, you've had your warning, that's it'") and poverty. "In the early Sixties, we were plunged into this Cathy Come Home thing. It wasn't glamorous to be a single parent. The old man never contacted us or paid a penny in maintenance."
Sent to a comprehensive, Chapman turned up only for English and drama classes. A stage school saved him. "We dragged the cash together. The fees were £60 a term, and I felt incredibly at home there."
Then an agent phoned Chapman and told him that the director Alan Clarke was casting a Borstal movie. "He sat me down and said, [Chapman's Merseyside accent is perfect] ''Ave you 'ad a chance to look at the script?' I said yes. He said, 'You know there's a scene in the greenhouse? We think you're the guy for that. How do you feel?' I said, 'I'll do it.'" He beams. "Imagine – the first job I get, I'm shafting someone up the arse in a greenhouse, then going back to school."
But he loved working with Clarke, whom he reveres for "his openness and generosity". The master of Northern realism was clearly a surrogate father.
Thus fledged, he found himself stuck in tough-guy roles for years: in Made in Britain, The Fourth Protocol, For Queen & Country, Gangster No 1, Hellbound, Hellraiser... Frankly, it was a relief to get to the National, playing a gay man dying of Aids.
Typecasting still bothers him: "I want to say, 'Look, there are other colours and other sizes, and if I can't play them, I won't play at all.'" A complex fellow, Chapman: a big, sensitive chap with a parent-sized hole in his heart, who, after 30 years playing tough guys, is now impersonating a man discovering his inner weeping woman.
Oh, and one of his two daughters has started work experience at the National, just as he's about to publish a novel warning actors of the frustration and insolvency of the profession. Themes of masculinity and parenthood seethe around his head, as they have seethed through his career, and now through the play he's in. No wonder he's so good in it.
'A Prayer for My Daughter' is at the Young Vic, London SE1 (020-7922 2922; www.youngvic.org),to 15 March