Dan picking up the cat before going into Frank's house was unrehearsed. Ryan Gosling saw the stray cat during the filming to the scene, and picked it up. Because Dan's cat died a few scenes before, the filmmakers thought it was perfect for the film and put it in the final cut.
Was originally made as the short film Gowanus, Brooklyn (2004), which won an award at the Sundance Film Festival. Shareeka Epps and Karen Chilton reprise their roles as Drey and Drey's mother. Matt Kerr, who plays the substitute teacher Mr. Light in this film, played Mr. Dunne in the original short.
Rachel McAdams came to visit the set during the filming of the bar scene. Running low on bar extras, to help the producers out Rachel hopped in to help make the bar look busy. Her scene never makes the final cut.
The film's title "Half Nelson" refers to a wrestling hold in which one arm is passed under the opponent's arm from behind to the back of the neck.
The Dan Dunne character was originally written to be in his mid-thirties. However, the filmmakers changed this to accommodate Ryan Gosling's age. Gosling was 25 years old at the time of filming.
Ryan Gosling received his first Academy Awards nomination in the 'Best Actor' category for this movie.
(According to the filmmakers' commentary on the DVD) the film was shot hand-held, except for a short sequence in Dan Dunne's apartment. The sequence in Dunne's apartment happens when he is not there, and it contains all static shots. These static shots were shot on a tripod.
The school used in the film (with the bright "Neon Celery" and "Key Lime" colored classroom doors), was painted by students from the school through an innovative nonprofit youth development program called Publicolor in NYC.
Although it was produced and shot in New York City, the Toronto band Broken Social Scene is featured prominently on the soundtrack. Broken Social Scene even recorded some original music for the film.