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)"and (Ray - "No its not its true" Helen "Yeah I dont think so Ray"). In real life, the two later fall in love and marry.
Both Sarah Michelle Gellar and Jennifer Love-Hewitt would go on to great fame in the horror genre, Gellar as 'Buffy the Vampire Slayer' and Hewitt as 'The Ghost Whisperer'. Buffy creator Joss Whedon designed the character as an antidote to films where the sexy blonde girl is always killed by the monster. In 'I Know What You Did Last Summer' Gellar plays just that part.
The set relocated from North Carolina to California for scene where the 4 teenagers run over the man. Producer Erik Feig said that North Carolina was the flattest state and they needed a more 'curvy' and 'dangerous looking' road.
The original trailer for the movie described Kevin Williamson and the movie as "from the creator of Scream (1996). Miramax, owners of the "Scream" franchise successfully sued Columbia Pictures for "false advertising" and the phrase was removed.
The film originally had a different ending, where Julie James is chatting with Ray online, she gets the pool party invitation, and then another person starts a chat with her typing in, "I still know." Then the killer jumps out on her. This ending was used as a teaser trailer for the sequel.
The trivia items below may give away important plot points.
Max's Murder was added in re-shoots. It was decided that the killer, who remained mostly in shadows for the first half of the film, worked on a suspense level; but never provided a tangible threat to the main characters. The murder was then filmed to show the audience that, indeed, this character was a deadly threat to the four leads.
Aside from the characters names and the teens hitting someone with their car, the plot is almost completely different for the original novel. In the novel they did not hit a fisherman but a young boy who was the brother of one of Helen's new friends. In addition, No one dies in the book. Barry is shot with a gun but survives and Julie comes to a close encounter and was almost choked by Collie, the boys brother, but Ray hits him with a club.
The last thing that Max says to Barry is that he is going to turn him in to the police. It is later established that the killer had followed the four leads to the fishing pier and had taken pictures. This implies that he had witnessed the scene where Barry threatens Max and finally learned that Max was present when the car accident happened. This provides a motive about the murder of Max, since the police would have taken Barry under custody for the assault against Max or at least would have watched him. Further investigation about the involvement of the five teens in the fatal night with possible links to the David Egan drowning case would have led to even more interrogations and undoubtedly the killer's plans would have been foiled since he had only a couple of days before the 4th of July. This explains why Max was dispatched right afterwards while the killer continued to stalk the other 4 leads knowing that they could not resort to the police.