For nine years, 202 episodes and most of the 1990s, Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) investigated paranormal phenomenon in The X-Files. To that list can be added unidentified flying objects, unexplained events, real-world monsters and a complex government conspiracy involving aliens and the abduction of Mulder's sister.

By the time filming of the series finished in 2002, Duchovny says he and Anderson were a little crazy.

"I think what happens is when you're doing a show like we did and there aren't that many of them, but you're talking about eight or nine years of working 12 to 14 hours a day with the same person ... I think we both went somewhat crazy," he says.

Under the watch of creator-producer Chris Carter, the series turned both actors, then unknowns, into international stars. That rapid fame and everything that went with took its toll, Duchovny says. "It's such a big change in someone's life ... It's not the way human beings should live, right?

"I think there were times when we were nuts with one another, where she was acting crazy or I was acting crazy, or we were both acting crazy. [And] the thing about being crazy is you don't know you're crazy. With time, I look back now and I say, `Oh, I was a little crazy.' And I think [Gillian] can do the same thing."

Although they returned for two feature films, the last of which, The X Files: I Want to Believe, was released in 2008, both shifted their acting focus away from genre. He became the wild, childish Hank Moody on the comedy-drama series Californication, and she Lady Dedlock in Bleak House and Miss Havisham in Great Expectations.

"I was in some ways reacting against The X-Files, trying to find something different," Duchovny says of the raunchy and funny Californication. "I was kind of despairing. There's no comedy for me to do, but I feel funny. Why can't I find that? Then [Californication] came along and I was like, `It's not a movie, but let's see what we've got. Let's see what we can do here.' "

For a time, it seemed The X-Files might simply be consigned to a place in the annals of television history. It had a loyal and loving audience, but there was no sustained campaign to bring the series back, perhaps because the chance of succeeding, with two otherwise engaged lead actors, seemed unusually slim.

First there was a whisper, and then it grew to a roar: 20th Century Fox confirmed it would dust off its much-prized franchise for a six-episode limited series. There is even talk that the studio wants more, although neither Duchovny's nor Anderson's schedules – he is starring in the crime drama Aquarius, she in British thriller series The Fall – would likely permit it.

In a post-Snowden, post-Wikileaks world, a redux of The X-Files almost seems mandatory, although, perhaps to the disappointment of fans who seek a glimmer of the character in the man, Duchovny is a sceptic.

"My own personal view on conspiracies is people can't keep secrets," he says. "I've never known anybody, one person to keep a secret. That was my problem with John F. Kennedy. There was no way. If there was a conspiracy ... somebody talks. Somebody along the way opens their mouth. There are too many people. There are too many secrets. It's not human nature."

Does the government keep secrets?

"Yeah, sure."

Are some of them worse than others?

"Sure, yeah. I think that's where I find the reality of it. Why would they be hiding this, if we believe that they could be hiding the state of alien technology that they're hoarding in order to use against us. That's good storytelling."

The rebirth of The X-Files is also proof, perhaps, that there are some characters in an actor's life who demand their own encore. "If you're lucky, people want to continue to see characters that you've done ... I find it a compliment that people want me to do something again, something that I've already done," he says.

Of all the roles he has played, Mulder holds a particular affection for the 55-year-old actor. "It was the beginning of everything for me, and I feel the show is so special that in some ways it never ... it doesn't leave me in some way. It kind of didn't leave the culture in some way. I don't know why, it just seemed that people would always talk about The X-Files forever. I don't know why."

Returning to the set – the limited series was filmed in Vancouver, where the show's first five seasons were filmed – the cast recaptured their rhythm almost immediately.

"At this point, it's kind of – cellular is the word you used – it's a good way to put it. We did so much work in the past. We did nine years of preparation.

"I think just showing up on set, and being with Chris, and sitting down across from Gillian for a scene, a lot of it started to come back there. There's a certain way that we act on the show."

At times, he caught old episodes of the series. Those glimpses evoked a mixture of feelings. "I see a lot of kind of growth that needs to happen, but I see a certain kind of eagerness that is also winning, that is impossible to fake," he says. "It's a strange and wonderful process to go through as I get to play it, as I get to continue to play it."

Among the original series' most notable hallmarks was its genre-lurching style. Its various writers, who included Howard Gordon (Homeland) and Vince Gilligan (Breaking Bad), wrote with very different tonalities.

"We had many genres within the show – horror, thriller, mythology," Duchovny says. "There's the romantic element that creeps in every now and then, and there's the comedic X-Files. Each one of those has a slightly different type of performance in it."

So, now that he has put it on the table, let's talk about the romantic element – that is, the unresolved sexual tension that simmered between Mulder and Scully for much of the original series. Viewers of the original series and its two spin-off movies will know it's a matter of historical record that the unresolved sexual tension was, ahem, resolved, but the new series does to some extent try to reset the parameters of the romance between Mulder and Scully.

"You've got to start somewhere to get somewhere, right?" he says. "It seems important to certain fans that Mulder and Scully be together romantically ... Stasis is not exciting dramatically, happiness is not exciting, dramatically. Good relationships are not exciting, dramatically, or comedically, or in any way really except in real life."

So what do you do?

"You want your show to move. You want to go from A to B. So I would think the fans would be more pleased knowing that if you start broken up, you're probably trending towards getting back together, rather than the other way around. Whatever is together is going to break, and whatever is broken is going to get mended. That's the nature of television or drama.

"We start broken, so you make the call ..."

Yet, after a moment of reflection, he hints that the limited series may not offer resolution on all its fronts.

"Chris would never conceive of any instalment as the end," he says. "I think he always wants to leave it open-ended, so even if we had gone into this going, `This is the end of it,' I don't know ...

"It's kind of a show that doesn't have a natural ending, because the nature of the show is to deal with possibility."