PLOT details for the new movie The X-Files: I Want to Believe were kept under such tight wraps by writers-producers Chris Carter and Frank Spotnitz that, for a while there, even David Duchovny did not have a copy of the script.

Duchovny, who returns to his character, former FBI agent Fox "Spooky" Mulder, in the new movie, first learned of the plot when Carter and Spotnitz came to his house, script in hand.

"They disappeared for about an hour and a half, two hours, and let me read it and then I came out and handed it back to them," Duchovny recalls.

"There was a while there where I couldn't even get my own copy. I had to go to Chris's office to work on the script and finally I said, 'You know, unless you want me showing up completely unprepared, you should probably let one go'."

Gillian Anderson, who returns as Mulder's former partner Agent Dana Scully, initially did not even get a hard copy of The X-Files: I Want to Believe in her hands.

"I had this very short window to read it on a computer," says Anderson, who now lives in London.

"I'm not even sure whether it was 100 per cent there at the time."

But Anderson regards as understandable all the secrecy that, despite the best efforts of bloggers, continues to surround the plot right up until the eve of the film's worldwide release this week.

"Chris has made something that he just very, very strongly believes is unique and special and he doesn't want it to be broken apart and bastardised in the sense of being thrown around all over the place," she says.

"It's about protecting something he and Frank feel needs respect, and the minute it starts to get out there and get distributed it loses some of that. It loses some of its intensity.

"They also want people to be genuinely surprised."

Perhaps one of the biggest surprises is that The X-Files package, the result of 202 hours of television (1993-2002) and a previous spin-off movie released in 1998, ever came back together.

When Duchovny, 47, quit the TV series after 173 episodes, he was suing Carter and the studio, 20th Century-Fox, over alleged discrepancies in his profit participation payments.

Carter, who created the series and also directs The X-Files: I Want to Believe, had wanted to do another movie soon after the TV series wrapped, but there was more legal manoeuvring between him and Fox. Anderson, 40 next month, is on record saying she would never have appeared in the final season of the series, in which Robert Patrick replaced Duchovny, had she not been forced over "a very big barrel" by the studio.

And, especially towards the end of their on-screen partnership, all was not meant to be well between Anderson and Duchovny personally.

Eventually 20th Century-Fox gave them all a choice – now or never.

If the two stars ever were at loggerheads, they are certainly not about to admit it now and, in fact, they insist I Want to Believe has been a harmonious reunion for them and Carter.

"I don't think Chris would have entered the project without one of us and I don't think either of us would have entered without the other two," Duchovny says.

"Aside from loyalty, we kind of treat each other as the keepers of a particular part of The X-Files. Gillian is the keeper of Scully and she knows how to do that and what it takes. I keep Mulder, and Chris kind of oversees the entire thing.

"I can't imagine me actually wanting to do it without one of the others. It just doesn't seem right to me." Anderson backs him up, saying: "When it was first discussed that we might do a film in the future, the agreement was that we'd only do it if it was right for all of us and made sense and didn't jeopardise other projects or our families.

"I've been on board since we first talked about it. It was just a matter of some of the other little bits and pieces that needed to be worked out along the way, I think."

Duchovny even suggests I Want to Believe might be the beginning of a new ongoing X-Files chapter.

"I was always thinking of this, from, like, the seventh year on," he says.

"I thought, 'Let's try to quit this at our height and then we'll jump over into a movie franchise'. We didn't do exactly that, but obviously the show did well enough that we're able to come back and do this now."

Duchovny, Anderson and Carter agreed to make the movie for a total budget of about $30 million, relatively modest in Hollywood terms.

"We acknowledge that it is not at the height of the television show, that there isn't the demand there might have been four or five years ago," Duchovny says.

"We all wanted to make the best movie we could and have all the money end up on the screen rather than take it into our pockets and we saw this almost like an audition for a movie franchise.

"It was, you know, 'Let's play by Fox's rules if this is the amount of money they're going to release to make this movie, and let's make a great movie and make it immensely successful'.

"It's kind of like we have to make do with what we've been given as an investment in the future of a franchise. Maybe then we'll get to go on and do what we like in the next one and then the one after that."

Since finishing The X-Files: I Want to Believe, Anderson has announced she and boyfriend Mark Griffiths are expecting their second child together. They have a son, Oscar, who will be two this year, and Anderson has a 13-year-old daughter, Piper, from a previous marriage.

Duchovny has been married to actor Tea Leoni since 1997. They have a daughter Madelaine, 8, and six-year-old son Kyd.

The X-Files: I Want to Believe opens in cinemas on Thursday.

Gillian Anderson	Danny Dyer (Straightheads/Closure/Vendetta a Due)

Billy Connolly (The X-Files Movie #2)	Andrew Davies (Sceneggiatore di "Bleak House")
Denis Lawson (Bleak House, 84 Charing Cross)	Piper Anderson-Klotz: qui.