The van ride to the Vancouver set of “The X-Files: I Want to Believe” feels a lot like what approaching Area 51 must feel like. A mystery awaits, although you’re not at all sure what kind of clues you’ll actually find.

The production assistant behind the wheel is perfectly friendly, talking about the weather (cold), the city (growing), and the best place for Chinese food (pretty much anywhere). But when the conversation takes a turn into X-Files territory, he quickly tucks today’s production schedule into his jacket and offers up more news about the weather.

Back during the show’s nine-year run on Fox, it was tough enough to unravel the mysteries behind the space aliens, black ooze and that guy who smoked an awful lot. Now, however, it’d be easier to have brunch with Osama bin Laden than to find out what is about to happen with FBI agents Fox Mulder and Dana Scully in the movie, out July 25. David Duchovny and Gillian Anderson are shooting on a nondescript set that might be an office, looking just as they did in 2002 when the series cam to an end. They’re talking through a scene that has something to do with a crisis of faith that at least one of them is having. But for a couple of agents dedicated to uncovering others’ secrets, they’re awfully tight-lipped when it comes to revealing a few of their own.

“Even my wife [Tea Leoni] doesn’t know anything about the story,” claims Duchovny when the scene ends. “When she sees the movie she’ll be completely surprised.” He pauses. “Well, she does know one very important thing I can’t talk about, which nobody should know.”

The equally cryptic Anderson adds only that this “particularly thoughtful and important scene” takes place “in a structure that is important to Mulder and Scully, and that brings a certain history.” Helpful, thanks.

Still, it’s not like there are Men in Black stalking the set, ready to abduct anyone who shares a single story secret. The mood is downright jovial. Producer-director-writer Chris Carter’s dog Larry roams free, looking for affection and snacks. Duchovny, his famously idiosyncratic sense of humor intact, wanders around with a pair of scissors and offers free haircuts (there are no takers). Anderson happily shows off pictures of her new addition, adorable toddler Oscar. (The actress is expecting another child later this year with Oscar’s father, businessman Mark Griffiths; she also has a 13-year-old daughter from her first marriage).

Everything seems so relaxed, it’s easy to forget that this whole production of the movie is basically a covert operation. “We are using any tactic we can to keep things quiet,” Carter explains, his voice barely louder than a whisper. “Nobody has been killed in the making of the movie. Yet.”

The tactics include requiring all but the two lead actors to turn in their scripts at the end of each day and allowing only a handful of crew members to read those pages (and even they had to do it in a room with video cameras on them).

“There was a day I thought I’d lost my script,” recalls Duchovny, kicking back in a director’s chair on set during a break. “I thought, “I’ve ruined it for everybody.’”

OK, maybe not everybody. But Carter figures that X-philes everywhere want going to the movie to be like opening a present on Christmas morning: The less you know about what you’re getting, the more fun it will be. A few stokcing stuffers have leaked out, though. For instance, the story is set I winter 2008, so it’s happening in real time, picking up with Mulder and Scully six years after we saw them.

“The emotional story of mulder and Scully is really the heart of the movie,” says producer-writer Frank Spotnitz. “It’s about their love story.”

It’s also a stand-alone experience, so anyone not steeped in The X-Files’ twisted mythology will still be able to follow along. There are two new FBI agents, played by Amanda Peet and rapper Xzibit, and Scottish actor Billy Connolly steps into priest robes. At some point in the story, reveals Duchovny obliquely, “I rescue Scully and put her in my parka and it looks so ridiculous—I looked like Kenny from South Park.” Holding up a bandaged hand, the actor also reveals that some things never change. “I would love to hit people, but true to Mulder form, I get hit a lot.” (Of course, this is Duchovny, so he’d actually like to start an untrue rumor while he’s at it: “That I am a wolfman. I wish it were true. I’d love that X-Files.”)

It was apparently a lot easier for Duchovny to slip back into his alter ego than it was for his costar. Anderson never doubted that Mulder and Scully would return for their second feature film (following 1998’s “The X-Files: Fight the Future”), but those first few days at work felt “unbelievably familiar and also freakishly foreign.”

“I had the worst first and second day,” says a frowning Anderson as she scarfs down a boxed lunch in her trailer. “But I realized I’ve spent such a long time since the show trying not to do anything that resembled Scully.” Thus, the actress moved to London after the series wrapped and appeared in several plays there an in indie movies such as “The Last King of Scotland.”

In the end, it took just a few minutes alongside her partner in crime fighting to bring Scully back. Says Anderson: “A couple times, I’ve looked over and gone, ‘Oh, wow! That’s Mulder standing next to me!’” She and Duchovny stayed in touch via e-mail since the series ended, but their first chance to catch up in person was a lunch just after the movie was becoming a reality. “It was the first time I’d seen David in ages,” she recalls. “It’s emotional. We have a lot of history. I spent more time with David than I have with most of my friends and some family members.”

For Duchovny, the initial script reading “was more sentimental than our actual reunion [over lunch]. I realized that Gillian and I have changed over the years, but when you start reading these people, they haven’t. They are the same and they feel the same way about each other.”

Right, but what way is that exactly? Duchovny grins, “There’s definitely some skin in this movie,” he says slowly. “And not all of it is being flayed or ripped apart. There’s some Gillian skin, and some of mine.”

Maybe that’s the movie’s Big Secret. This time around, it’ll be more like “The X-Rated Files.” There will be no more clues from Duchovny or Anderson, though. As they head back to shoot a scene, the weather-obsessed driver has returned to escort prying eyes home. Yeah the truth is probably out there but you’ll have to see the movie to find it.