There are quite a few things fans don't know about Gillian Anderson. For the most part, they know that she's currently playing Margaret Thatcher on The Crown, but they may not know that she still has a complicated relationship with her X-Files co-star, David Duchovny.

While Gillian Anderson seems to have a certain reputation around Hollywood, David Duchovny does too. In fact, there's a theory that his Californication character (Hank Moody) was based on him. Therefore, we can imagine these two Hollywood titans clashing... And apparently, they did.

Let's take a look at why these two feuded and what they feuded about...

It Started To Come To Light In 1997

Fox's The X-Files ran from 1993 until 2002, had a revival from 2016 to 2018, and spawned two films in 1998 and 2008. Throughout the show's run, David Duchovny and Gillian Anderson were locked in a mysterious feud that neither has been 100% honest about.

According to Metro UK, David blames the sheer amount of hours they worked together for their feud: "Familiarity breeds contempt". Then he said, "We used to argue about nothing. We couldn’t stand the sight of each other." But not everyone who spends time with someone begins to hate them... So, what really happened?

According to UpRoxx, fans started to catch wind of their feud in 1997 when Gillian forgot to thank David or X-Files' creator Chris Carter during her Emmy win for the role. Afterward, she took out a full-page ad in Variety thanking them and the cast and crew.

However, a year earlier, David Duchovny was quoted in a book talking about the tension he had with his X-Files co-star: "We don't hang out. We are very wary of the fact that at any moment the other can turn into a psychotic human being because of the demands that are put on us, the sixteen-hour days. So I know when she is tired and irritable, and she knows the same about me. We have a great respect for the fine line the other is walking all the time."

Yet, when David won a Golden Globe two years later, he praised Gillian as an amazing co-star... Yet multiple interviews he gave around the win suggested that he was supremely frustrated with Gillian.

It's all very confusing.

What is clear is that the two had very different working styles. David, who hated filming the show in Vancouver, wanted to move at a faster pace. Gillian, on the other hand, was a perfectionist and took forever to get ready and wanted to constantly redo her work.

Related: Gillian Anderson Ditches Pants To Promote ‘The Crown’

David And Gillian Further Complicated The Narrative On Jimmy Kimmel

While promoting their X-Files revival on Jimmy Kimmel Live back in 2016, David Duchovny and Gillian Anderson were asked about their complicated history. Both of them seemed fairly uncomfortable when answering this line of questions... Although, both of them didn't exactly seem thrilled to be sitting there with each other in the first place. But they moved through it all as best they could... They are professionals, after all.

"Today...," Jimmy Kimmel began. "I was very surprised at how friendly you guys were and are because I was under the impression that you guys didn't like each other when you were shooting the show."

"You mean friendly to each other?" Gillian asked.

"Yeah."

Then the three of them started to joke about how Jimmy Kimmel and his producers made the two X-Files stars engage in a personal and touchy skit before they came on to interview.

"But even off-camera. There was a lot of comradery it seems there, " Jimmy said.

"There is," Gillian claimed.

"Mm. Mhm," David nodded.

"Was it always like that?" Jimmy asked, causing both of them to shake their heads...

"No," Gillian said.

"So, I do remember that correctly. There was some rough patches."

"Sure, yeah," David said, clearly not wanting to get into the subject.

This is when Jimmy probed more, asking what exactly the two X-Files icons would fight about on and off of the set.

"Like, if you had to pick one thing," Jimmy continued, seeing that both David and Gillian were both physically squirming in their seats. "What was it that rubbed you the wrong way about each other."

"Well, I wondered this for a long time and, uh, um... I think part of it, you know, we shot in Vancouver [British Columbia, Canada] and it's very moist in Vancouver," Gillian said of the wetter climate the stunning coastal city often has in the Spring, Fall, and Winter.

"It was the humidity?" Jimmy asked, laughing.

At this point in the interview, Gillian couldn't get it together. Between laughing about claiming Vancouver was "moist" and his uncomfortable she appeared to be with the line of questioning, the whole thing turned into a s*** show. David, on the other hand, played off the tension with his typical Hank Moody expressions.

"Let me just say, I have no idea where it's going. And It's kind of fun," David said. "I would finish the story if I had any idea what you were talking about. But I don't."

Related: Who Is ‘X-Files’ Star David Duchovny’s Girlfriend?

As she got it together, Gillian claimed that the moistness in the Vancouver air made her hair very frizzy...

"That's what it was!" David joked.

"It takes us forever. Between every single take, they would have to blowdry my hair again. And things take a long time," Gillian said.

"And I got pissed at that?" David asked.

"Well, I think it added to the tension..." Gillian said.

"It kind of makes me sound like an a***hole."

Their Relationship Is Still An Enigma

The last time we heard from either David or Gillian about their relationship was in 2018. According to The Daily Mail, Gillian was asked about her somewhat mysterious feud with David. This is when she said she actually didn't really know him... even though they've worked together on-and-off for decades...

"We’ve spent so much time with each other over the years that I’ve probably been together with him more than in any other relationship I’ve had. But that doesn’t necessarily make you close. We might have a little chit-chat between scenes but we don’t really talk about our personal lives because we’re at work. And we don’t have meals together because we’re spending so much time in each other’s company."

While both X-Files' stars have been coy about what's really happened between them, it's pretty obvious that their relationship was anything but a walk in the park.

Next: Here’s What ‘X-Files’ Actress Yvonne Boismier Phillips Is Up To Now