The pair - who have become close friends since they first worked together on the 2010 comedy Grown Ups - play teachers in the action comedy about a high school teacher, played by Kevin, who turns to cage fighting to raise money to save the music department.
Salma revealed of their kissing scene: "I wanted a double, because I'm really good friends with his wife and children. I mean, I've been kissing people on screen all the time, but we're good friends and the kids are good friends. It was the strangest thing... He's actually a very good kisser!"
She added: "It was through a fence, nobody opened their mouth. But he does have good lips..."
Meanwhile, 47-year-old Kevin was disappointed all his fight training didn't have more of an effect on his appearance.
Kevin said: "It was horrible, horrible. And you know what the most horrific thing about it is, you train for so long and you're still fat! I just ate raw for eight months of my life and I'm still jiggling."
:: Here Comes The Boom is released in cinemas on Friday, November 9.