Kevin Williamson wrote this script before his screenplay for Scream (1996), but was unable to sell it. Following the big screen success of "Scream," Columbia Pictures immediately bought I Know What You Did Last Summer (1997) and produced it to capitalize on the newfound horror success. Because "I Know" is mostly a straight-up, modern update of the old slasher horror movie rather than a deconstruction of the genre, many critics and fans felt this was a huge step down from Williamson's more clever and innovative Scream scripts.
Despite being part of the main cast and appearing in many key scenes together, the characters Helen (Sarah Michelle Gellar) and Ray (Freddie Prinze Jr.) only speak to each other twice (Ray - "How do you know that?" Helen - "Missy said there was a friend named Billy Blue)" (at around 1h 2 mins) and (Ray - "No, it's not, it's true" Helen "Yeah, I don't think so, Ray") (at around 10 mins). Despite the fact that the two barely interact onscreen, Gellar and Prinze became good friends after meeting on this film. They later fell in love and married.
The ice inside the storage of the boat was made of gelatin so Jennifer Love Hewitt could comfortably move around inside.
Writer Lois Duncan was vehemently opposed to her young adult novel being reworked into a slasher film, stating she was "appalled" in one interview. This was due to the fact that her youngest daughter was murdered by an unknown assailant in 1989. She said, "As the mother of a murdered child, I don't find violent death something to squeal and giggle about." Her original book wasn't that far off from a slasher though: it's still a bunch of guilty teenagers being stalked and terrorized by a murderous madman. It's just that nobody dies in her version, so in that sense it's more like an old school mystery.
Jennifer Love Hewitt had just appeared along former Scream Queen Jamie Lee Curtis in House Arrest (1997) prior to filming this movie and was surprised to see she was also filming a movie in North Carolina at the same time. Jennifer said Jamie came over to the soundstage to wish her luck on her first horror film and would come over every day after that to give her a hug.