Heavenly Creatures, a movie from the 90s which I LOVED when it came out. I'm pleased to say it holds up.
Heavenly Creatures is about two teenage girls who develop an extremely intense friendship built on a shared, symbiotic fantasy life. The friendship is all-consuming and sexual, and each girls' family is concerned.
When the family of one of the pair decides to relocate out of the country, the girls, immersed in fantasy, decide to kill the other's mother (this isn't a spoiler -- the murder is plain at the beginning). As Pauline writes in her diary, she understands why she and Juliette are not understood by those around them: "it's because we are both stark, raving mad!"
The director did a brilliant job rendering the fantasies, and the acting by the entire cast was terrific. The film features a very young Kate Winslet (one of my favorite actresses), who is the charismatic one, and the actress playing the sullen and odd Pauline was wonderfully creepy.