Everyone remembers Saturday Night Fever as this cheery disco romp, but the 1977 film is actually pretty bleak, the flashy dance routines standing at odds with a remarkably grim story about working-class Italian-American men who take out their rage on the local Puerto Rican community when they’re not busy trying to rape the women in their lives. (It’s a fun disco movie in the same way that It’s a Wonderful Life is a heartwarming Christmas movie, which is to say that it’s not. At all.) Not to say that it’s bad; it’s just really, really dark. I’ve read conflicting things about the musical adaptation, running through September 8 at Theatre by the Sea: that it tones down the racism and sex and violence, but also that it adheres pretty rigidly to the screenplay. By all accounts, this one’s going to hinge on the choreography and the chemistry of the two leads.