The scene where Louise grabs Thelma's headphones from her and scares her was not planned. In the DVD's audio commentary, Geena Davis commented where she was supposed to get up when Susan Sarandon called her from the car, but had the volume on her Walkman turned up too high, and did not hear her cue, so Susan came over to get her.
The sequence where Darryl slips and falls over on the builder's supplies as he is leaving for work was unscripted, as Christopher McDonald genuinely lost his footing. Despite this, he remained in character, yelling at the workmen as he got into the car and drove away. As he states on the DVD commentary, director Sir Ridley Scott liked the result so much, he kept it in the film.
It took producers so long to find someone for the role of Louise that Geena Davis had to sign a contract stating that the producers could cast her in either role if need be.
Before Brad Pitt was cast, the role went to William Baldwin but he ended up dropping out. George Clooney was rejected multiple times. Mark Ruffalo auditioned. Eventually, the role went to Robert Downey Jr. but Ridley deemed him too short compared to Geena. Geena was the one who picked Brad to do the part.
Sir Ridley Scott was reportedly so amazed with Hans Zimmer's score he created a main title sequence (with Zimmer's music over it), rather than giving the main credits at the end, as it was primarily planned.