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.
As a thank you for putting him back into the A list with this film, Robert Downey Jr. pushed for Shane Black to be involved in the Marvel franchise - he ultimately wrote and directed Iron Man 3 (2013).