Ever since co-writing and starring in Good Will Hunting Matt Damon has enjoyed a meteoric rise to success in films such as Saving Private Ryan, The Talented Mr Ripley, Ocean's 11, The Bourne Identity and, most recently, The Departed. With the character of Jason Bourne, he helped to revamp the spy genre and left James Bond shaken and stirred enough to re-invent the 007 franchise. The actor tells BBC Movies why the Bourne films have been so successful and, on the eve of third film The Bourne Ultimatum, why it might not necessarily be his final adventure...

Q. Did you always intend to make multiple Bourne movies?

A. No, I signed on for one at a time because I didn't know how it was going to go, whether or not I was going to like working in that genre, or whether there would even be another one. There were a lot of things that could have happened. But we always said after the first one that we'd do another one if it wasn't just a cynical money grabber; if we could actually make a movie that we felt was as good as or better than the previous one. With both the two subsequent films that was what we said and going into the third it was only that Paul [Greengrass] wanted to direct it that was my cue to sign up. He didn't need to direct a third Bourne film because he's about as white hot as you can get by Hollywood standards, given United 93 and everything that happened with that film, and The Bourne Supremacy and everything that happened with that. So when he said that he wanted to, and that it would be a laugh and some fun, we all went "OK".

What do you like about working with Paul Greengrass?

Well, his style really is helpful. The very first day that I worked with Paul [on Supremacy] in Russia we were shooting in this tunnel and we were losing the light. Usually when that happens people start to panic a little bit. We had to get this shot where Bourne is walking though about a hundred extras, he's just been shot, there's someone chasing him and he has to check where the bullet has passed from his back through his shoulder and check for blood on his finger. So I said to the A camera operator: "Hey, what's your bottom frame so I can show where the blood is, because if it were up to me I'd check for the blood down at my waist." But if he was cutting the shot higher I'd have to bring my hand higher.

Paul heard this and came running over from the other side of the tunnel and said: "No, no, no, absolutely not. You just do what you do. And if it's low then we'll go down low and if you don't see the blood then don't worry about it, we'll know what it was." That was my first day working with him and that is basically the best thing you can hear as an actor, a director saying: "I want you to do this as honestly and truthfully as you possibly can and I will capture it. You have no responsibility to me or to my frame whatsoever." The experience of that whole movie and this one was the same.

How did you find shooting in a crowded place like Waterloo or Tangier?

I think it all helps with the urgency and the paranoia of the performance, because we're right out there in the middle of everything. A lot comes out of really being put in locations. I was actually reminded this morning about the rooftop in Tangier and the sequence where I grab these towels and use them to jump over this wall that has glass on it.

We got up there and there was just glass on the walls and we had to get around that somehow, so we set up a clothes line and said: "Why don't we grab one [a towel] from this roof and then go a couple more rooftops and grab another thing..." There's thousands of things like this that we come up with on the day and we just leave the good ones in the movie. [Laughs].

How hard did you have to train physically for this film?

For me actually it was different than the other two. I used to not have a life at all. I would go in work all day, then go home and go to the gym and then go to sleep. I'd do that every day until the movie was over. But now I have a stepdaughter, I have a daughter and a life at home, so I didn't go to the gym after work.

I showed up in really good shape ready to go. I was wearing that jacket and by the end of the movie that zipper was creeping up higher and higher. Paul actually had to cut around it at the end because I looked at a rough cut and said: "Remember that scene? Maybe we could use a tight shot there!" [Laughs]

Will there be a fourth Bourne movie?

Well, you're going to have to ask Paul Greengrass because if Paul did it then that would be a case for me doing it...

Are the rumours about your appearance in the new Star Trek movie true?

I hear those rumours too but usually at press junkets, so I ultimately called JJ Abrams to ask what the deal was. But he said I was way too old. He said: "We're doing Kirk and Spock from when they get out of the flight academy so it's guys that are 15 years younger than you playing the roles.&quot

You were recently named by Forbes magazine as Hollywood's best value star. Considering the banter that took place between yourself and your Ocean's co-stars Brad Pitt and George Clooney when they were named the world's sexiest stars, did it give you some bragging rights?

Oh yeah. In fact, the email that went out to George said: "Suck it sexy boy!" He wrote back: "That's great. I had that read to me by one of my servants!" But those things are pretty arbitrary. It's great for me because in the short term it means I get some freedom to choose the movies that I want to do. But if you look at the list in five years time, who knows what it would say. I don't know any actors who take a job based on how well they think it's going to perform at the box office.

I don't know which movies the list was drawing from. Of the movies that I've done since Bourne Supremacy, none of them would have been commercial on the face of them - they were Syriana, The Departed and The Good Shepherd. All three of them were just the best scripts that I'd read.

I'm pretty sure that Syriana and The Good Shepherd ended up in the black. And while The Departed ended up being a big hit, classically Martin Scorsese's movies don't make any money; even the absolute masterpieces like Raging Bull and Goodfellas. They're not huge hits in the way that a Spider-Man movie is. So, when you take a movie like that you take it knowing that it's probably not going to be a good box office and all the actors cut their fee just for the honour of working with him. But it was because of the Bourne movies that I had the freedom in the last two years to do those kinds of movies that I thought had wonderful scripts. I didn't even give a second thought to what they would or wouldn't do at the box office.

What kind of extras can we expect from The Bourne Ultimatum DVD?

Well, one of the DVDs came out and said there were explosive extra scenes. But it was all these scenes that weren't good enough to be in the movie [laughs]. So, I went to the studio and said: "That's kind of like walking into a house holding dog mess and saying 'hey, look what I almost stepped in!'" But I've seen the extras for this DVD and they're very good. It's more behind-the-scenes than anything.

The Bourne Ultimatum opens in UK cinemas on Thursday, 16th August 2007.