Both Leigh Brackett and Robert Altman have said that Sterling Hayden and Elliott Gould's dialogue during the drinking scenes was improvised. This was because Hayden was drunk and stoned on marijuana most of the time.
Elliott Gould has said that so long as he is physically able he holds out hopes that he could reprise the role of Phillip Marlowe. He has a screenplay entitled "It's Always Now," based on a Raymond Chandler story, "The Curtain." The Chandler estate sold him the rights to the story for $1.
Elliott Gould improvised the scene in police custody in which he smears fingerprint ink all over his face.
Robert Altman decided that the camera should never stop moving, and put it on a dolly. However, the camera movements would counter the actions of the characters so that the audience would feel like a voyeur.