In the film, which drew enthusiastic applause at a press screening, the US actress plays an ageing courtesan who seduces a rival's callow son, two decades after she starred in Frears' erotic drama "Dangerous Liaisons."
She told a press conference full of questions on how she maintains her still-striking beauty that the film industry was not such a bad place for an actress of a certain age.
"They do allow you to get older in Hollywood. Some of us continue to work and this is a good example of it," said Pfeiffer, who will be 51 in April. "It seems that my leading men just keep getting younger the older I get. But lucky for me, I don't really mind it," she purred.
She said she had learned a few tricks about the business in her 30 years of acting.
"When I'm working I take really good care of myself and when I'm not working I stay out of sight," Pfeiffer said. "And I stopped smoking when I was 30, that helped."
"Cheri" is based on the 1920 novel by French writer Colette and adapted for the screen by Christopher Hampton, who also wrote "Dangerous Liaisons." It tells the story of Lea, a still beautiful, independent woman of means thanks to her illustrious career as a concubine to princes and industrialists, who is living in Paris on the eve of World War I.
Her former colleague in the business of seduction - Madame Peloux, played by Oscar-winner Kathy Bates - complains to her that her handsome young son seems content to live a life of debauchery with no direction or purpose. So Lea takes "Cheri," as she calls the 21-year-old, under her wing - and into her bed.
Asked about the prevalence of May-December romances on screen at the festival with an older woman on top, including "The Reader" starring Kate Winslet as a woman in postwar Germany who takes a teenage lover, Pfeiffer said with a smile: "I think it's a positive step in the right direction."
Pfeiffer said she was thrilled when Frears asked to work with her again. "She worked out what fading beauty might be like because she clearly doesn't know anything about that," Frears quipped.
The director bristled when a reporter asked whether a tragic romance set in Belle Epoque France was appropriate during a global economic slowdown or at the Berlin festival, which is known for showcasing gritty political cinema.
"Of course in the Depression, people used to go to see Fred Astaire films," he said, noting that audiences often sought "escapism" in tough times. "I can show you a miserable film if you want."
Frears said it was not easy to strike the right tone in telling the story of a seemingly superficial affair between two wealthy, frivolous people that turns sour.
"There's a tragedy going on underneath, that's why it was so difficult, that was what you had to get right," he said. "And it goes right back to Colette, that was how she wrote the book."
"Cheri" is one of 18 contenders for the Berlin festival's Golden Bear top prize, to be awarded by jury president Tilda Swinton at a gala ceremony Saturday. The event wraps up the next day.