There is at least one secret Gillian Anderson can reveal about "The X-Files: I Want to Believe."
For Anderson, returning to play Agent Dana Scully, the role that made the 39-year-old actress famous, is what was really scary.
"I found it quite tricky over the first couple of days to slide back into it," says Anderson. "It was kind of unexpected and a bit freakish for me."
Freakish, that is, until the first scene that reunited her with Duchovny, aka Agent Fox Mulder. It was innocuous - just a brief conversation between the two agents as they walked down the hallway at FBI headquarters.
But the old magic was back.
"It was definitely the moment that [we] got into the same room together that it felt familiar again," says Anderson.
For Duchovny, the chance to revisit Mulder was more like slipping on a comfortable old trench coat.
It was a far cry from the way he left "The X-Files," burnt out after nine seasons of spending 10 months a year starting his week at dawn on Monday and ending it in the early morning hours of Saturday.
"I missed the heroic quality of [Mulder] - he's got a hell of a lot of integrity and he's a dreamer," says Duchovny. "And sometimes he made me a better person, just because he was so single-minded and courageous."
So when director Chris Carter, the show's creator, called to ask if they wanted to do a second "X-Files" movie, both Duchovny and Anderson leaped at the chance. The first, 1998's "The X-Files: Fight the Future," earned $187 million at the box office worldwide.
Almost everything else about the film is shrouded in the kind of secrecy usually reserved for vast government conspiracies.
During filming, Carter and the producers had copies of the full script. The actors were only given pages related to the scenes they were shooting at the time, and they were collected and shredded afterward. And in the spirit of the show's mantra, "Trust no one," fake images were leaked on the Internet to throw spoiler sites off the scent.
"I'm already looking over my shoulder [just talking to you]," quips Duchovny.
But the biggest mystery is whether, after six years, the fans still want to believe.
Many X-philes are still recovering from the horror of the way their beloved show ended in 2002. A contract dispute kept Duchovny out most of the last season, and the show buckled under the weight of its confusing back story about a government/alien conspiracy.
"Nobody wanted to see Duchovny leaving the show, nobody wanted substitutes ... nobody really liked the direction of the last season," says Harry Knowles, founder of movie fan site Ain't It Cool News. "With the mythology I just think they dropped the ball."
Duchovny bristles at such talk. To him, the show should be remembered for scaring the bejesus out of America for nine seasons.
"I'm just amazed it could stay good for that long," he says. "I'm not at all embarrassed or ambivalent about it petering out. I guess when we were ending, we kind of wished people had applauded our longevity rather than saying, 'Well, what happened there? It just ended?' But it didn't happen that way."
Since the end of the series, a lot has changed for Anderson. She has moved on - literally, from Los Angeles to London - performing onstage, in "Bleak House" on "Masterpiece Theatre" and in critically acclaimed films like 2006's "The Last King of Scotland."
Next up is a movie about Martha Gellhorn, the war correspondent and wife of Ernest Hemingway, in which Anderson will produce and star. And in the next three months, she will find time for the birth of her third child.
"I don't think anyone would have paid me any mind at all if I didn't have ["The X-Files"] behind me," says Anderson.
After making his directorial debut with the 2004 film "House of D," Duchovnyfound himself back on the small screen, this time as the star of Showtime's "Californication." Duchovny plays Hank Moody, a frustrated writer who gets more action in the first minutes of the pilot than Mulder got in nine years - not counting a tryst with a vampire.
"It's funny, people's attitude toward sex is so loaded - if you play a murderer, it's rare that somebody will say, 'What does your wife think?'" says Duchovny, when asked about the reaction of his spouse, actress Téa Leoni, to watching her husband's weekly sex scenes.
If "X-Files: I Want to Believe" scares up enough box office, both Duchovny and Anderson say they are ready to return for more sequels. If not, then at least they had a chance to say goodbye to the franchise that helped launch their careers.
"We're all willing to jump in there again if that's what's called for," says Anderson. "Or to let it go."