Phantom of the Paradise (1974)
Brian De Palma wrote and directed this update of the classic tale, featuring Paul Williams as the evil record producer Swan. Williams composed all the music and lyrics for the soundtrack and also supplied the singing voice for the titular character.
The Rocky Horror Picture Show (1975)
A midnight-movie phenomenon in the 1970s, its popularity and influence endures to this day. "Time Warp" is a perfect entry point; that's Barry Bostwick and Susan Sarandon as the befuddled couple.
Little Shop of Horrors (1986)
The sadistic Steve Martin introduces himself to the audience in "Dentist Song," which is basically every nightmare you've ever had about dentists rolled into one hilarious sequence.
Sweeney Todd: The Demon Barber of Fleet Street (2007)
Tim Burton's adaptation of Stephen Sondheim's Broadway musical gave Johnny Depp the opportunity to sing "My Friends," an ode to his deadly razors, in a duet with Helena Bonham Carter.
Stage Fright (2014)
A giddy spirit races through a delightful rendition of "We're Gay," as new arrivals at a summer camp for music theater geeks express their true feelings through song and dance. The movie pokes fun with numbers like this, but later the comedy turns quite dark and explicitly violent.