writer and creator Chris Carter says it was "like the old days" when the cult TV show's stars Gillian Anderson and David Duchovny returned to filming.
FBI special agents Fox Mulder (Duchovny) and Dana Scully (Anderson) are back for another six-episode series, nearly 23 years after the sci-fi series first aired.
"It was like the old days, and we all hit the ground running," the screenwriter said.
"People want to know what that first day was like, getting back together - you would imagine that it would be a lot of smiles and back-slapping.
"I'm telling you, we had so much work to do on that first day, that no one had a chance to even, I think, recognise that personal, 'We're back together after eight years apart'."
The X-Files originally ran for nine seasons until it ended in 2002, and led to two feature films - with the last one in 2008.
Since the end of The X-Files, Anderson has gone on to star in The Fall, while Duchovny has been in Californication and Aquarius.
But the paranormal drama has been revived with a special "event series" and Carter said he had kept Mulder and Scully in mind over the years.
He explained: "I think about it all the time. I've been doing this now for nearly 25 years, so these characters are in my blood."When I pick up the newspaper, I almost always see something that could work as an X-Files episode."
The mini-series opens with Mulder and Scully no longer in a relationship.
"I'm honest to where I believe they would be in their lives, based on all that we have established in the mythology," Carter said.
"And in the second movie, where they were together, I thought naturally they would arrive at a bump in the road, based on what their characters are, and their individual pursuits."
The return of The X-Files has stirred up hope among fans of the show, who are keen for more than a six-episode comeback.
"It's funny, it's not, like, not enough that you're coming back. It's like, are you coming back for even more?" Carter said."I think it's a natural thing, it's like - are we going to go on a first date, are we going to go on a second date?
"Fox did ask me a question about more, but it was sort of like an off-the-cuff thing, and they were wondering if there were going to be more, when that could be.
"I had no good answer for them because I don't know. But I think that everyone is waiting to see how we do in the ratings."
The "abbreviated season" will end with a cliffhanger.
"We don't want to tie up all the loose ends, it might suggest that we're done with the show," Carter said."But I'll leave it as a question: are we done, are we not?"