- Josh Hartnett (Slevin) lived with writer Jason Smilovic and his girlfriend in New York while the script was being written. Smilovic said that he thought of having Slevin wear a towel a lot of the time because he saw Hartnett in one so often. More so it added a vulnerable quality to Slevin.
- The hand X-ray in the coroner's office is the director's hand.
- The name of the film comes from an episode title of "The New Shmoo" (1979). Shmoo is also referenced by Morgan Freeman in the movie. However, Morgan Freeman's description of a Shmoo is a paraphrase of the standard version from Li'l Abner by Al Capp who introduced the fantasy beast in 1948.
- On promotional material for the film, Ben Kingsley's credit included his KCVO title. At first, the actor was singled out for some criticism as such things are usually omitted from professional credits. It transpired that this was a mistake by a studio executive, who was unfamiliar with the British honours system.
- According to the DVD commentary with Josh Hartnett and Lucy Liu, Lindsey's reaction when she walks in on Slevin with his towel open is real. The scene was scripted but Josh Hartnett actually flashed Lucy Liu without telling her prior.
- Lucy Lui said in a making of feature that she had more fun making this movie than any of her other movies.
>>> WARNING: Here Be Spoilers <<<
Trivia items below here contain information that may give away important plot points. You may not want to read any further if you've not already seen this title.
- SPOILER: An alternate scene was filmed in which Slevin kills Lindsey but was considered too dark.
- SPOILER: (Warning! Also contains a spoiler for House of Sand and Fog (2003)): It is the second movie in which Ben Kingsley dies by having a plastic bag over his head. The other one was House of Sand and Fog (2003), although in that case, he did it to himself.
- SPOILER: In the end, the pseudonym alias of Slevin; "Kelevra" which is Hebrew for 'bad dog' ties with Bruce Willis's character, Smith/Mr. Goodkat.
Related Links