Dawson Casting: The majority of the students were played by actors in their early twenties. This is averted by Veronica and Heather Duke—Winona Ryder and Shannen Doherty were both 17. Christian Slater just about gets by, being nineteen at the time. Most of the extras are in their late 30s (look close).
The original ending had Veronica take J.D.'s bomb and blow up herself and everyone in school, followed by a surreal sequence where everyone who died is shown in the afterlife at a prom, where everyone — regardless of what clique they were in when they were living, if they were in a clique at all — gets along. This was regarded as "too dark" by executives and it was changed to one where Veronica rejects J.D.'s ideals and J.D. kills himself with the bomb..
Executives also changed Heather's posthumous underlined copy of Catcher In The Rye to Moby-Dick, due to the notorious real-life killers associated with Catcher. (And due to Salinger's refusal to relinquish the rights). The word "Eskimo" does not appear in Moby Dick for Heather to have underlined (though "Esqimaux" appears), whereas Catcher has the following section about the Natural History Museum:
The best thing, though, in that museum was that everything always stayed right where it was. Nobody'd move. You could go there a hundred times, and that Eskimo would still be just finished catching those two fish, the birds would still be on their way south, the deers would still be drinking out of that water hole, with their pretty antlers and their pretty, skinny legs, and that squaw with the naked bosom would still be weaving that same blanket. Nobody'd be different. The only thing that would be different would be you.
The leads were almost played by Jennifer Connelly and Brad Pitt. Connelly turned down the role to audition for Say Anything (which she lost out to Ione Skye) and Pitt lost out after Slater had a better audition. Heather Graham was also considered for Heather Chandler, until her mother vetoed the script due to the swearing.
A script for a TV version was pitched to the then-fledgling Fox Network. They had to decide between a TV adaptation of Heathers and some teen show called "Beverly Hills High" (though you might recognize it by its more popular name: Beverly Hills 90210).