Seth Rogen cannot help seeing the funny side of life. "I don't try to find it, it just happens," he says. "I can't remember the number of times somebody's been telling me a movie idea they have and I think it's a comedy, and it's not. Someone'll go: 'There's this guy who's hit by a car and he has to get his leg cut off', and I'll go: 'That sounds hilarious!' It's just how my brain hears things."

Given this admission, it is not really surprising that Rogen, best known for starring in the 2007 hit movie, Knocked Up, is one of the creative forces behind a new comedy, 50/50, about a young man (played by Joseph Gordon-Levitt) facing an uncertain future after developing a malignant tumour of the spine.

Even before the movie had been screened, some people rounded on Rogen - who plays Levitt's boorish but loyal friend, Kyle - online, offended by the notion that anyone could think there was a funny side to living with cancer. Of course, in the wrong hands 50/50 could easily have gone horribly wrong. As it turns out, director Jonathan Levine's brave film is a finely-balanced combination of tears and laughter.

That the film works is in large part due to the fact that it is rooted in the real-life experience of its writer, Will Reiser, who, in 2005, was diagnosed with cancer.

Reiser has been part of the "Jew Tang Clan" - a group of young Jewish entertainers centred on Rogen and Knocked Up director Judd Apatow - since he met Rogen on the US version of Sacha Baron Cohen's Da Ali G Show eights years ago. Currently in remission from the disease, he looks healthy but stick-thin sitting on a sofa in a London hotel suite next to his round-faced friend.

Reiser recalls how 50/50 started simply as a discussion at someone's birthday party -- two months after he had learned he was gravely ill - about how a lot of people's impression of cancer comes from movies. "They're usually really morbid and sad, and they're usually about people who are middle-aged and misanthropes, and they always die in the end," Reiser says. "But that had nothing to do with our experience."

"Especially not that night," Rogen laughs. "We were at a bar trying to hit on girls, essentially. And that was nothing we had ever seen in a movie before."

They jested about making their own film. "Although, I wasn't really joking," says Rogen, who had seen an opportunity to do something new. "I was kind of serious about it." And they were not short of things to write about. "There was so many absurd, ridiculous things that were going on in my life," says Reiser, "that it just felt like there was this wealth of material that we could use."

The story would have been very different if his initial diagnosis of low-grade lymphoma - a terminal condition - had not been incorrect. "Having a giant tumour in the spine seemed like a victory after that, to be honest," says Rogen. "It was a lot easier to be optimistic and joke about it then."

Gordon-Levitt's character is given a 50 per cent chance of survival after surgery. In Reiser's case, the odds were not so specific. "It was unclear as to how big the tumour was, so they didn't know what the outcome would be," he says.

"So it was easier to trick yourself into thinking it was going to be totally fine," suggests Rogen.

The idea of laughing one's way through a crisis seems like a particularly (some might say, stereotypically) Jewish perspective. Rogen, who performed stand-up comedy at a Habonim Dror camp as a teenager, and who once even wrote jokes for a mohel, is not so sure however. "People always want to make it seem like we invented humour in the face of adversity. And we might have," he says. "But I know a lot of non-Jewish people who are funny in the face of adversity as well, so I can't take religious ownership over that. But I would like to."

For Reiser, pouring his thoughts and emotions into a screenplay turned out to be a powerful way of dealing with feelings he was unable to articulate, even to his close friend. "I wrote a movie instead, that's how I dealt with it. And it turned out to be the best form of therapy."

To their surprise, the film has had the same effect on viewers, who have found in it a way to open up about their own experiences of the disease. "People just feel like it's really relatable," says Reiser. "Yeah," jokes Rogen, unable to help himself. "What Witness did for Amish people, this has done for cancer survivors."

Inevitably, having cancer changed Reiser. Before his diagnosis, he was a "neurotic mess", he admits. "I worried about everything and I complained, and I was over-sensitive and I was insecure. Just through the process of getting better and getting past it, I kind of let go of a lot of that and I'm a much more laid-back person now."

Rogen also came out of it a different person. The same night they floated the idea of making a film, Reiser introduced his friend to actress Lauren Miller (who has a brief appearance in 50/50). Last month, the pair married in a ceremony conducted by a female rabbi in northern California. Does this mean that, like Kyle in the movie, Rogen used Reiser's illness to get a girl?

"Not with Lauren," says Reiser. "But my having cancer definitely made him look more sympathetic, like a very compassionate friend."

"I wouldn't say it made me look compassionate," Rogen pitches in, laughing again. "She got to see how truly compassionate I am."

Now that 50/50 is done, the friends are once again mining Reiser's life for a follow-up. This one will be based on a trip he took to Jamaica with his grandmother when he was 14. "Unbeknown to any in my family, she was in the very early stages of Alzheimer's," Reiser says, "and I actually lost her in one of the most dangerous cities in the world."

"So we're making that into a comedy as well," Rogen giggles.

'50/50' goes on release today