Lea Thompson originally turned down the lead when she was approached by Howard Deutch. She then accepted the role after Howard the Duck flopped, and Howard Deutch again asked her to play the role of Amanda Jones.
The three main characters have names relating to The Rolling Stones: Amanda Jones, named after a song of the same name (which is played in the film); a drummer called Watts (Charlie Watts) and a character called Keith (Keith Richards.
When Martha Coolidge was hired to direct, she cast 'Kim Delaney' as Amanda Jones and Kyle MacLachlan as Hardy Jenns. When Coolidge dropped out before filming was to begin and Howard Deutch replaced her, he replaced Delaney with Lea Thompson (after Eric Stoltz suggested her for the role) and replaced MacLachlan with Craig Sheffer.
Elias Koteas was discovered by John Hughes auditioning for another one of his films and it was at his suggestion that he would be cast in the role of Duncan.
The car that Watts drives in the movie is a very dilapidated mini cooper modified for driving in America. Between 1960 and 1967 only 10,000 were sold in the U.S. and then discontinued. Although they are very popular now, they were quite alien and unknown to most of the American public in 1987.
Most of the extras in the film that play the "bad kids" in the detention scene and around campus weren't thugs at all. They were football players from the various high schools in the area. They were chosen mostly because they could tower over the actors and look menacing.
Several years before this movie was filmed, Eric Stoltz had been the original choice to play Marty McFly in Back to the Future, and he had even shot five weeks' worth of footage as Marty before he was replaced by Michael J. Fox. Lea Thompson, who plays one of Stoltz's love interests in Some Kind of Wonderful, played his would-be character's mother in Back to the Future.