"I forgot it was Matt and me after about 10 minutes and then I forgot it was two guys," 68-year-old Douglas, who stars in HBO's Behind the Candelabra, told PEOPLE.
"Both of us are in long-standing marriages, so that makes it easier. It's like love scenes with a girl. If you're an actor, you gotta do it. We read the script; we knew what we had to do. The challenge was making it as real as possible. But I would tease Matt about which flavor lip gloss he'd like for me to use!"
In a wide-reaching interview from the Cannes Film Festival where he was promoting the film, cancer survivor Douglas reflected on how he needed "some good news" by way of the Liberace project after his health battles and "major issues with my older son."
"He's a great person," Douglas insisted, noting that he regularly writes to his son.
Douglas is scheduled to return to New York within days where he'll also reunite with wife, Catherine Zeta-Jones, 43, who has been released from a facility where she was being treated for bipolar II disorder.
"Bipolar is a slippery path and I think something you don't think you need any meds, and you get away from them, and all of a sudden, whoa," Douglas told the publication.
"She's doing a really good job of getting a 10,000-mile checkup. She's doing fine and I'm proud of her. She's doing a really good job of getting balanced."