Is “Robert Downey Jr.” a cheater? Do you have to fake an orgasm with “Sean Penn”? Is “Alec Baldwin” a psychiatrist hog?

Yes — according to Amy Sohn’s fictional characterizations of these celebs in her splashy new novel “Prospect Park West.”

The saucy book, out Sept. 1, is quickly becoming the talk of the town not just for its outrageous, yet nearly spot-on parodies of smug, smothering Park Slope parents and the culture of the cult-like Park Slope Food Co-op — but also for its liberal use of real celebrities as walking, talking characters.

The book creates a scathing portrait of Park Slope’s mommy brigade — of which Sohn is a breast-feeding member — as a parade of unsatisfied thirty- and forty-something moms sizing up their plights relative to all the other stroller-pushers at the playground. Few are having sex — at least not with their spouses.