Welsh actress Catherine, 43, told The Sun that she was flattered that Matt imagined he was kissing her while shooting the scenes, instead of her husband.
She said: "I must say I have seen Liberace: Behind The Candelabra, and if my husband is going to kiss anybody else I'm so happy it's Matt Damon.
"Supposedly, when Matt was kissing Michael, he said that he closed his eyes and pretended he was kissing me, rather than Michael, which is very flattering.
"Matt said that to Michael. Matt was closing his eyes, kissing Michael but pretending he was kissing me.
"I thought that was one of the biggest compliments I have ever had. Or so f****d up."
Catherine, who previously suffered from bipolar disorder, plays a psychiatrist in new movie thriller Side Effects.
She said: "Just from the experiences that I have seen and been part of, there has been this British, 'stiff upper lip' towards psychotherapy and depression that basically meant your emotions were quashed...
"'You're sad? Well, buck up. You'll be fine tomorrow.' But now it is starting to come out much more in the open and people are starting to talk more about it, on television, about issues."