Because of that, shooting sex scenes with the actress was especially tough.
"[Filming the sex scenes] was torture, yes it was," Gyllenhaal laughed. "But it was also not torture. I mean, come on, it was like a mix of both."
"Weirdly, love scenes are awkward, because there are maybe 30, 50 people watching it? That doesn't turn me on," he explained. "It's oddly mechanical. And also it's a dance, you're choreographing for a camera. You can get in it, but it's like a fight scene, you have to choreograph those scenes."
The 40-year-old actor also shared the pair used the "pillow technique" while filming, a method that involves placing a pillow in between the actors when it comes time to roll on those steamy sex scenes.
"The pillow technique was used," he said. "That was just preemptive and used generally always when actually in a horizontal place in that movie. I think that was actually a Jennifer suggestion. She was very kind to suggest it before we began. She was like, 'I'm putting a pillow here.'"
This isn't the first time Gyllenhaal has opened up about his crush on the former co-star. In 2016, the "Brokeback Mountain" actor told PEOPLE his secret infatuation lasted years.
"She's a rough one, you know, not likable. So hard to compliment," he sarcastically told the publication at the time. "I will say, I had a crush on her for years. And working with her was not easy .. I was – um, yeah. That's all I'm going to say. It was lovely. It wasn't hard, that's what I would say."
According to Gyllenhaal, it wasn't watching her performance on "Friends" that sparked his interest, but the way she carried herself that caught his attention.
"Not so much 'Friends', but kind of her personality from afar, and movies she was in," he admitted. "And some of 'Friends' – I'm not a huge 'Friends' fan – but who she is."
While Aniston may have been his on-set crush, in real life he's been with model Jeanne Cadieu for years. The two just made their red carpet debut at the New York Film Festival last week.
"It was nice having her there," he told Starn. "It was great."
When asked about marriage, Gyllenhaal said "there's only so far I'll go as, you know, in talking about" their relationship publicly. "I love her so much. She's such a good person," he added. "I mean, I don't think she really enjoys, you know, all the other stuff ... and so that's part of the reason why I just adore her. And not at all the main reason but, you know, one of them. I mean, it was great to have her there."