Los Angeles - Cameron Diaz "barely worked out" before stripping off in scenes for The Other Woman.
The 41-year-old actress, who is seen running down a beach in a bikini with Sports Illustrated model Kate Upton, 21, in the new movie, admits she wasn't worried about her figure and being filmed with an actress half her age.
Talking on 2Day FM radio station, she explained: "When I did that scene I was barely working out.
"I wasn't like 'Oh my God, I am going to be in a bikini and Kate is going to be in a bikini and what is everyone going to think of me.
"I am 20 years older than her. I am not trying to be her. She is a beautiful woman. We are completely different and I celebrate her."
The blonde star also insists she doesn't wish to be 21 anymore and she feels "great" about her body, adding: "I am very happy to be 41 and I love it."
Kate - who joins Leslie Mann on screen for comedy The Other Woman - has become friends with Cameron after they gossiped about ex-boyfriends together.
Asked who is more likely to call an ex when they're drunk, Cameron, who is single and previously dated Justin Timberlake, replied: "I don't have anybody I want to drink dial."
And Kate, whose former boyfriends include Dancing With The Stars professional Maksim Chmerkovskiy, added: "I don't have anyone I want to talk to. They're an ex for a reason. I'm like, 'Okay, and go! Next!"