Michelle Pfeiffer gives the "cougar" concept a whole new meaning in the 1920s period fantasy 'Cheri,' in theaters June 26, and ET has her take on romancing a sexy younger man -- two decades younger -- onscreen!

"[My onscreen love interests are getting] younger and younger -- I never would have predicted," Michelle tells ET. "Lucky for me [Rupert Friend] is just incredibly smart, and really, really funny and a really fun person to be around. I enjoyed him very much."

So, how does Michelle stay looking so youthful at 51?

"You look at all these magic bullets for weight loss and this and that, and the truth of the matter is we don't want to take the time to work out, or we don't want to take the time to eat healthy and just lead a healthy lifestyle -- which ultimately is what you have to do," she says.

"All of those other things have crazy side effects, and are maybe a temporary fix, but we don't really want to know about that," she continues. "I haven't found any of that stuff to work. Somebody said to me once, 'If your grandmother used it, you can consider it kind of safe.' I use that as my barometer."

An adaptation of the 1920s French novel by Colette, 'Cheri' finds Michelle as an older woman grappling with middle age who teaches a wealthy young man (Rupert) in the ways of love. But when he is forced to end the relationship after marrying someone else, he retreats into a fantasy world.

"She's a courtesan, which I guess would be the equivalent to a modern-day call girl," explains Michelle. "When you say that, there are certain images that come to mind, and certain qualities and behaviors, and she was so different than the stereotypical kind of prostitute, really."

Also starring Kathy Bates and Felicity Jones, 'Cheri' was directed by Stephen Frears ('The Queen,' 'The Grifters').

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