As a show of support for Robert Downey Jr.'s recovery from alcohol and drugs, Val Kilmer refused to drink during the entire production.
Val Kilmer met Robert Downey Jr. for the first time at a Hollywood party. A week later, he received the screenplay for this film, and agreed to do it before he'd even finished reading it. Upon agreeing, and much to his delight, he was informed that Downey had already been cast.
The film was originally titled "L.A.P.I.", then "Bang!", but Val Kilmer suggested to director Shane Black that "Kiss Kiss Bang Bang" would have more appeal.
Robert Downey Jr. considered his role as Harry Lockhart his "calling card" to playing Tony Stark/Iron Man in the Marvel Cinematic Universe. While preparing to make Iron Man (2008), Downey Jr.'s performance inspired director Jon Favreau to cast him in the title role.