The actress told Entertainment Weekly, "I think we’ve made a clear statement that we keep certain things just between us. It was very easy to unite, but that's quite personal."
Fassbender, 39, who is starring alongside his girlfriend in their new film "The Light Between Oceans," out Sept. 2, also explained how they've become such a private couple while staying in Hollywood's spotlight.
"I'm not going to talk about my private life with a total stranger, unless I feel like I need to," he said. "Why would I? I don't."
Still, the actor understands why the media and public are hungry for more information. He can personally relate.
"I might have been curious about actors' lives when I was growing up. That's human nature," he admitted. "We're all curious about a lot of things. But my curiosity didn't obligate them to tell me.
"It's the worst thing if you're sitting there in the theater, going, 'Oh, that's the guy who dates this person and likes to do this in the morning and that in the afternoon.' Then you're just watching a brand, as opposed to an actor," Fassbender added.
Vikander, 27, said she simply prefers to "know less about the actors I look up to."
"That's what I mean -- I love the mystery behind it all," she said.