Josh Hartnett (Slevin) lived with screenwriter Jason Smilovic and his girlfriend in New York City 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.
(at around 23 mins) 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 to filming.
Although meant to be around the same age in the movie, Lucy Liu (Lindsay) is 10 years older than her on-screen co-star Josh Hartnett (Slevin).
Lucy Liu said in a "Making of" featurette that she had more fun making this movie than any of her other movies.
(at around 22 mins) When Lindsey first meets Slevin she says, "According to Columbo, there are three things to look for at a crime scene--What's there now that wasn't there before, what was there before that isn't there now, and what's been moved." During the entire run of Columbo (1971) from 1968-2003 he never says anything similar to this.