The original script for this movie was actually a lot funnier than the one they filmed. Director Howard Deutch was having trouble casting for the original script, and was sacked. John Hughes then rewrote the script to be more dramatic, and eventually Deutch was rehired as the director.
Part of the reason the original, more comedic script wasn't filmed was that Michael J. Fox turned it down. Ironically, the part then went to Eric Stoltz, who was the original casting choice for Back to the Future before being replaced by Michael J. Fox.
The part of Amanda was originally intended for Molly Ringwald, in what would have marked the fourth collaboration between her and Hughes. However, Ringwald decided to sever ties with Hughes in order to avoid being identified too closely with him.
Kim Delaney was then chosen for the part, but Deutch replaced Delaney with his future wife Lea Thompson when he was rehired as director. Whether Stoltz would have appeared as the male lead without Thompson is another question (Thomas F. Wilson has claimed that Stoltz had a thing for Thompson during the filming of Back to the Future).