Michael Fassbender said the on-screen chemistry was "there pretty much immediately" when he fell in love with girlfriend Alicia Vikander on the set of their new film.
The duo play an Australian lighthouse keeper and his wife who find a baby girl in a boat and decide to raise her in The Light Between Oceans, based on M L Stedman's best-seller.
Talking to Access Hollywood about their chemistry, Michael, 39, joked that it was a three-way thing that also included director Derek Cianfrance, who enjoyed getting up close with the stars during their love scenes.
"We had Derek, he's never behind the monitor," said Alicia.
"He's under the bed, saying, 'That's excellent, that's excellent'," added Michael, as Alicia laughed.
He continued: "It's always awkward because there's always people in the room, unless you're into that. It's always an awkward thing, you just have to create an intimate atmosphere in a non-intimate scenario."
The co-stars have kept their relationship under-the-radar since falling in love while shooting the film.
They have rarely been pictured together but the two went together to this year's Oscars and Michael was seen congratulating his girlfriend with a kiss as she picked up best supporting actress for The Danish Girl.
While Michael said that it was nice to "protect" their private moments, Alicia was a little more gushing about her beau in the interview.
She said, "I got to work with one of the absolute best actors we have and that made it a lot more easy", to which a bashful Michael replied, "Gee, thanks."
The Light Between Oceans is expected to be released in the UK in November.