But it wasn't just the spooky plots that had viewers hooked.
The X-Files owed much of its success to the chemistry between Mulder and Scully as they forged a deep bond despite their differences, united against the dark forces above them.
but right now it's really good.'I think the coolest things we do are the quintessentially Mulder and Scully moments, the scenes in Mulder's office when we're discussing cases where he's throwing pencils or eating sunflower seeds - they're a lot of fun.'But the whole thing was a little more challenging than I'd expected,' she admits.
She's also starred in high-profile TV series such as Great Expectations and The Fall, and can currently be seen as society schemer Anna Pavlovna in the BBC's War And Peace.
But both agree that, even after all these years, a return to The X-Files set felt like coming home.'In many ways The X-Files never left me - much as I would have wanted it to,' says David, 55.
For all the other work I've done, there's never been a time when people stopped talking to me about The X-Files, so being Mulder again felt pretty seamless.'Even so, he says there were hurdles to cross, not least reacquainting himself with the tongue-twisting scientific explanations Mulder became famous for.
'I got to the set on the first day and I was immediately given five pages of one of those horrendous monologues, where I had to say things like "electrogravitic propulsion".
'I think there was a point when I was doing the original show that I was afraid it was going to be the only thing I'd ever do or ever be known for doing.
But these days I see it as one of those once-in-a-lifetime roles that I'm completely grateful for.
And when their platonic relationship developed into a romantic one in series seven it was to the unbridled joy of the show's fans, known as X-Philes.
The last time we saw them, in 2008 spin-off film I Want To Believe, they were still together.
The show ran for nine seasons between 19 (it first aired here in early 1994), won 16 Emmy Awards and five Golden Globes and spawned two feature films - and next month the much-awaited new series will begin, complete with the original stars, David Duchovny as Mulder and Gillian Anderson as Scully, as well as Mitch Pileggi as their supervisor Walter Skinner and William B Davis as their nemesis, the mysterious Smoking Man.
The series followed the pair as they investigated the FBI's secret X-Files - unsolved cases involving unexplained phenomena or apparent paranormal activity - giving us one of TV's most memorable catchphrases 'The truth is out there'.
'Although I'm not really sure whether Scully and I were ever actually married,' he adds teasingly. But I think we were, and a man should know these things...