"I never thought about it ending," he says. "We always had the desire to grow the TV show into a movie franchise. We loved the show and we enjoyed working with one another. We owe so much to the characters, to each other and to the fans of the show, and we wanted to keep on doing that."

Gillian Anderson, who is six months' pregnant with her third child, agreed to do the film without so much as a glance at the finished script.

"My decision to be on board was made a long time before the script was even written. I don't know what I would have done if the script was bad, but fortunately I didn't have to face that.

"We'd had the discussion early on, that were it ever to come to fruition, we'd all be on board for another film. With six years having passed when we'd been doing other things, it felt like good timing."

The massively successful X-Files series spawned an earlier movie and ran for nine seasons, with viewers in more than 60 countries glued to Mulder and Scully's investigations into unsolved cases involving paranormal phenomena.

But shooting the series for 10 months a year took its toll on David, who left after eight seasons.

"I had to leave because of the time commitment. It was never 'God, I hate the show! I can't stand working with these people!' It was always just a case of 'let's stop the TV show now, because we're all tired and we've all worked longer than anybody's ever worked on a drama together,'

"You could say there were dramas that went on longer, but not a two-person drama with the same two people. We never changed. We did eight years of the same thing. It was really hard for me because I didn't really want to be there. I was miserable."

However, David was keen to step back into Mulder's shoes for the film, with both agents having left the FBI. Scully – now working as a doctor – and Mulder are living together. But their relationship is tested when FBI agent Dakota Whitney (Amanda Peet) asks Mulder to help investigate the disappearance of a female agent, with the help of psychic and convicted paedophile, Father Joseph (Billy Connolly).

Gillian says: "What makes the film unique is the particular kind of relationship between the two characters.

"It's almost a marriage in a sense. Even though we're not working side by side throughout the whole film, there's that intensity and the thriller storyline at the same time."

Despite the actors having rarely been in contact in recent years, they both admit they were able to recreate their characters' special relationship without much effort.

"The chemistry between David and I is something that is there naturally," Gillian says.

"It has been there despite ourselves, no matter what moods we're in or what was going on in our personal lives.

"No matter how we felt on a particular day, when we were in a scene together, the chemistry was there. There are shades of it in other projects I've done. But there's something different with David and that's the inexplicable bit."

David adds: "Because we'd had so little contact, there was an element of missing one another and missing working together, that was at play early on in the shooting.

"We had to trust that whatever used to work would continue to work. It's my feeling that we acted as a couple on the television show in all but physical intimacy.

"We bickered like a couple, we needed each other like a couple. It's hard to work on chemistry. It's either there or it isn't. It's like trying to fall in love with somebody. You can try, but if it's not there, it's not there."

Since The X-Files ended, both David and Gillian have distanced themselves from Mulder and Scully with a variety of different projects. Gillian impressed in Dickensian BBC drama Bleak House while David returned to TV in cult US hit Californication.

"When I left The X-Files, TV was all-consuming and it felt like a box," says David.

"But six years later, I've done a bunch of other stuff that I've enjoyed, and proved to myself as much as anybody else that I can do other things, so it's not really an issue anymore.

"If you were to ask me, 'Would you go back to doing the television show for X Files?' I would say no, because the time commitment was crazy."

The past 10 years have seen David become a parent for the first time – he has two children with his actress wife Tea Leoni, Madelaine, nine, and Kyd, seven. Gillian has one daughter Piper, 13, from her first marriage, and a 20-month-old son Oscar with partner Mark Griffiths.

"Having children changes your life tremendously, and in ways that, no matter what anybody speaks about, you never know until you're in the midst of it yourself," says Gillian, who lives in London.

"I've got friends who are completely paralysed by it and friends who behave as if it's just a tiny bump in the road. I think I'm somewhere in the middle."

It seems unlikely that, if The X-Files: I Want to Believe does well at the box office, fans will have to wait another six years to see more of their favourite FBI agents. But will David and Gillian be opening the X-Files again?

"We'll see..." says David. "It works and can work forever, as long as the stories are good." That sounds promising.

"But six years later, I've done a bunch of other stuff that I've enjoyed, and proved to myself as much as anybody else that I can do other things, so it's not really an issue anymore.

"If you were to ask me, 'Would you go back to doing the television show for X Files?' I would say no, because the time commitment was crazy."

The past 10 years have seen David become a parent for the first time – he has two children with his actress wife Tea Leoni, Madelaine, nine, and Kyd, seven. Gillian has one daughter Piper, 13, from her first marriage, and a 20-month-old son Oscar with partner Mark Griffiths.

"Having children changes your life tremendously, and in ways that, no matter what anybody speaks about, you never know until you're in the midst of it yourself," says Gillian, who lives in London.

"I've got friends who are completely paralysed by it and friends who behave as if it's just a tiny bump in the road. I think I'm somewhere in the middle."

It seems unlikely that, if The X-Files: I Want to Believe does well at the box office, fans will have to wait another six years to see more of their favourite FBI agents. But will David and Gillian be opening the X-Files again?

"We'll see..." says David. "It works and can work forever, as long as the stories are good." That sounds promising.

X FILES FACTS

The show first hit our screens in September 1993 and was so popular it became the first TV show to be released on DVD.

FBI agents Mulder (Duchovny) and Scully (Anderson) take opposing roles with Mulder being the "believer" and Scully the "sceptic".

The first X Files film was released in 1998, as a continuation of the season five finale. Series six picked up where the film left off.

Mulder and Scully have a strong on-screen chemistry but their relationship is strictly platonic in the series.

In real life, David is married to actress Tea Leoni and twice-divorced Gillian is now living with businessman Mark Griffiths in the UK.

The X-Files has won numerous Emmy and a Golden Globe awards, with Gillian winning one of each for best actress in a TV drama series and David picked up a Golden Globe for best actor.