CILLIAN Murphy knows what it's like to be ruthless. If killing a friend was the only way to save mankind, what would you do? "You'd kill him, " says Murphy. "It's the only logical choice." He's not speaking as himself, of course. He's a physicist on a last-ditch space mission to reignite the fading sun with a massive nuclear bomb. Due to an accident on board, oxygen is running out. Some of the crew must be jettisoned if the target is to be reached on time.

Danny Boyle's metaphysical thriller Sunshine is not the first time Murphy has faced such a choice. In The Wind That Shakes The Barley he had to decide whether or not to shoot an informer. "It's challenging to try and get inside that mindset and find out what having such thoughts floating around in your head does to you, " he says.

To help him in Sunshine he got to hang out with physicists involved in operating the Cern project in Geneva. It involves the construction of a gigantic particle accelerator that has a circumference of 27km. "I'm not quite sure what could happen when we turn this thing on, " Dr Brian Cox told him.

"It could create a black hole but it doesn't matter. One morning you might be mowing your lawn and then . . . SNAP! . . . you're gone. There wouldn't be any pain involved."

By rooting Sunshine in science (although in fact the sun isn't likely to burn out for another five billion years . . . the narrative just "accelerates" it a little) Boyle imbues Sunshine with a sense of awe comparable to 2001: A Space Odyssey or Tarkovsky's Solaris. "It was exciting to come to work and be discussing such profound issues, " says Murphy. "I think scientists are very aware of their insignificance in terms of the larger scale of things. What I found totally astonishing was that they quite happily say we don't know what 95% of the universe is, we just know what 5% of it is."

Perhaps that's also the secret of any lasting relationship, to not know everything about one another? "Well, 95% is quite a lot, " laughs Murphy, now almost 30, who married his longterm girlfriend Yvonne McGuinness in Provence just over two years ago.

"Give me another 20% to play with, maybe. It certainly keeps you curious. That's ultimately what drives a lot of these men and women."

Murphy, who grew up in Douglas in Cork, became an actor by accident. "There was no acting in my family, just a lot of music. They were all teachers. But that's a form of performance, I suppose." He studied to be a lawyer at UCC, playing guitar in a band in his spare time.

Although he had no training as an actor, he had a go at an audition for Corcadorca, a young company that he'd heard was putting on adventurous stuff. Next thing he was appearing at the Dublin Theatre Festival with Breda Walsh . . .and winning the Critic's Award at Edinburgh Festival . . . in Enda Walsh's two-hander Disco Pigs, playing the role of a teenager locked in an invented baby world, which he reprised with chilling effect in Kirsten Sheridan's movie version.

"I think if we'd been aware it would come to something, we wouldn't have been so relaxed or sure of ourselves, " he says. "It was a big brave act for Enda, casting someone like me with no experience."

Murphy was encouraged to go on by John Crowley, then establishing himself as a theatre director in London. "I looked him up there when I was 19 or 20 because I was from Cork and so was he. He gave me as much advice as he could. To be so helpful to a young fella who had done nothing was really generous." Several years later Crowley cast Murphy in Intermission, which became the most successful independent movie in Irish box-office history until it was topped in 2006 by The Wind That Shakes The Barley.

Last year Murphy starred with Neve Campbell in Crowley'sWest End production of John Kolvenbach's Love Song. "You have to have people to go to for advice in this game, " says Murphy.

Having starred in Danny Boyle's sci-fi thriller 28 Days Later, Murphy was eager to get together with him again. "But Sunshine came to me in the traditional way through my agent. I read it and did an audition for Danny and it worked out.

Danny is the sort of director who believes in very careful casting, as does Ken Loach. Even though he knows what you can do, he has to be 100% certain that you fit in the context of what he is imagining."

Murphy went into pre-production on Sunshine the weekend after he finished filming The Wind That Shakes The Barley. "From shooting Black and Tans in the hills of Cork near where I grew up to hurtling through outer space, " he says. "I love those sudden switches. Film roles are so transient. They come and they go. You've got to grab them when they're there. Out of hundreds of scripts you get each year, only about four are worthwhile and two of those are already gone to Leonardo Di Caprio. So you really have to fight for the two that are left. You're terrified of dropping the bar in terms of quality."

Two days before his wedding, Murphy flew to Los Angeles and back to convince Wes Craven he was right for the part of the psychopath in Red Eye. "I was struck by his energy, his face and his blue eyes, " says Craven. Back in 1999, long before the finance was in place for Breakfast On Pluto, Neil Jordan tested Murphy for the cross-dressing role of Kitten.

"Cillian was always obsessed with the role and kept asking me when I was going to make the film, " says Jordan. "Eventually he said, 'you know in a couple of years I'm going to be too old for the role, ' which led me to take another look at the script and then begin to try to get it made."

Although Sunshine has the look of a $150m blockbuster, it was shot in the small Three Mills Studios in the East End of London. "Danny is fearless, " says Murphy. "He'll take on the big Hollywood studios in terms of scale and themes, while having the freedom of working outside the box." Both Boyle, who grew up in Liverpool, and Murphy were brought up as Catholics.

Boyle even considered becoming a priest, before losing his faith, which perhaps explains why his films starting with Trainspotting create drama out of challenging moral dilemmas.

Murphy seems to have his feet more on the ground. "I don't recall wanting to save the world at school, " says Murphy. "I was never into the whole astronaut thing either." He points out that while Sunshine has a spiritual dimension, "it ticks all the thriller boxes along the way, so people can watch it for that as well".

The CGI effects in Sunshine gave Murphy his first experience of working with green screen. "I didn't have any of that stuff in Batman Begins, it was all done in post-production. Danny tried to make it as real as possible for us. There's a scene where we're looking at the sun and Mercury is orbiting it. We were just sitting there on the set but Danny rigged up this enormous curtain of reflective material and shone lights at it to bring it alive for us. He wanted everything to be as real as possible. He devised a helmet with a camera built into it to catch us close up when we were in our space suits. It was a beast of a thing. It took a huge effort to move about in it. A lot of the exertion and sweat you see was genuine. That's Danny's style of film making. It's very hard to act adrenalinised. You have to be adrenalised. I enjoy doing that."

Invariably Murphy opts to play outsiders or loners. "It's interesting to see what happens to the human condition in extremis rather than in your ordinary average run-of-the-mill life, " he says.

"To be put under pressure or . . . like in Barley . . . forced to do things you never conceived of before."

He'll next be seen in Watching the Detectives, a comedy which gets its title from an Elvis Costello song. "My character is a film buff who works in a video store. Lucy Liu is a femme fatale who comes into his life, and then hilarity ensues. For a change, I'm not a character in extremis."

He feels no need to move to LA.

"Most movies are cast out of LA and London simultaneously and many of them are shot here.

Besides, I'm European. I'm more comfortable in the European environment."

Halfway through filming SunshineMurphy became a father with the birth of his son Malachy.

"It helped that we were shooting in the East End, just a short drive away, " he says. "I got to change nappies. Afterwards I managed to take an eight-month break, just walking and talking and hanging out with him."

'Sunshine' opens on Friday