The elusive Hollywood couple rarely talks about their two-year romance.
The actors, who are playing husband and wife in the period drama, opened up about their film in an interview with the Entertainment Weekly.
Fassbender says it was not hard for him to seperate fact from fiction on the set.
"It wasn't the first time in a movie either of us had played somebody who is falling in love. There is an element of separation there. If I'm playing a murderer, I don't go out and start murdering people," he said.
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Fassbender, 39, and Vikander, 27, first met while filming the Derek Cianfrance-directed movie in 2014.
They made their awards-show debut as a couple at the Golden Globes in January.
Fassbender said he does not feel any obligation to talk about his love life.
"I'm not going to talk about my private life with a total stranger, unless I feel like I need to. Why would I? I don't.