Los Angeles - Matt Damon was baffled by the prospect of filming a sex scene with Michael Douglas.
The 42-year-old actor couldn't work out how he was going to convincingly pretend to make love to his co-star in Behind the Candelabra because he had never seen an X-rated scene involving two men.
He joked: "Michael Douglas and I were really curious about how director Steven Soderbergh was going to film the sex scenes. In the script, it just said, 'Scott makes love to Lee'.
"We asked him, 'Thanks for the note, but how do we actually do that?' Eventually, Steven put a camera under the bed, so you only see us partially and it never looks pornographic."
The star wasn't nervous about showing off his derriere to play Liberace's lover Scott Thorson in the movie, and was pleased to have matched Michael's infamous bottom-baring scene in Basic Instinct.
Asked what it was like to hop into bed with Michael, Matt laughed: "I finally know what Sharon Stone (Basic Instinct) and Glenn Close (Fatal Attraction) felt. Everyone remembers seeing Michael's ass in Basic Instinct...
"This time, you see mine, it's a bit blurry but in prime position in the middle of the screen, with a Brazilian tan line... It's that level of detail that makes your career!"
The Elysium hunk says the most difficult part of shooting the film - aside from the outlandish costumes and wigs - was watching the 68-year-old veteran actor shoot flamboyant pianist Liberace's losing battle with AIDS, since Michael had suffered from life-threatening throat cancer himself.
Matt told Madame Figaro magazine: "The reason he pulled it off so amazingly is because he'd acquired the wisdom, depth and distance of someone who has known illness and beaten it."