Jill Hennessy's twin sister, Jacqueline Hennessy, once played her sister's character during courtroom scenes filmed while Jill was unavailable, due to filming an appearance on Homicide: Life on the Street (1993).
The last of the original cast members to depart the show was Chris Noth. Although Steven Hill was often perceived as an original cast member, he joined the cast after the pilot episode. Noth's character Detective Mike Logan was written out at the end of season 5 because creator Dick Wolf felt that Logan and his partner Det. Lennie Briscoe had developed personality styles that were too similar. Noth did return to the L&O universe as Det. Logan in the TV movie Exiled (1998) (1998) and then again in Law & Order: Criminal Intent (2001), where Logan became a regular character for three seasons from 2005-2008.
Steven Zirnkilton, who narrates the opening credit sequences, only appeared on-screen once. He played a detective in the pilot episode, Everybody's Favorite Bagman (1990). He had one line of dialogue: "Look at that. Do you believe these guys?".
The distinctive "thunk-thunk" sound effect used in between scenes was created by combining close to a dozen sounds, including that of a group of monks stamping on a floor. The sound is intended to be reminiscent of both a judge's gavel and a jail cell door slamming.
Prior to becoming an actor, Dennis Farina (Detective Joe Fontana) was a Detective in the Chicago Police Department for eighteen years. He got into acting through his former partner and police commander, Chuck Adamson, who had retired and became a police consultant on movies and television shows.