By William Stevenson
Updated April 23, 1999 at 04:00 AM EDT

Dietrich fans who weren’t old enough to see her cabaret act in the late ’60s may want to check out this piece of celebrity worship starring Sian Phillips, but they’d be better off renting one of Dietrich’s movies instead. Pam Gems’ play is only so-so; the backstage drama in Marlene largely consists of the star telling her beleaguered assistants (Margaret Whitton and Mary Diveny in underwritten roles) about old Hollywood, World War II, and her famous lovers — from Frank Sinatra to Ernest Hemingway. Fortunately, Phillips (I, Claudius) is wonderful, portraying the legend as both a demanding diva and a vulnerable, aging exile gallantly trying to sustain the illusion of glamour. Her impersonation is so convincing that as the play winds down, with a half-hour mini-concert, you’d swear Dietrich had risen from the grave. As the Blue Angel herself would put it, she’s ”gweat.” B-