Years after this film was made, Paul Newman and Shelley Winters appeared together as guests on The Tonight Show. When Johnny Carson asked Winters, "Have you and Paul ever worked together?", Winters replied, "No, we haven't had the opportunity." Newman did an astonished double-take. "We haven't? What was I in Detective's Story (1966)? Chopped liver?" To her chagrin, Winters admitted that she had forgotten about the movie. Newman was incredulous. "I made love to you for two days, in front of the cameras! you tell me you forgot about that."
The opening credits sequence: William Goldman later said he knew he would succeed as a screenwriter as soon as he wrote the opening scene in Harper (1966) in which Harper has to resort to re-brewing coffee grounds from the trash can for his morning cup of coffee. Harper's dismay created empathy between the character and the coffee drinkers in the audience. Interestingly, that opening sequence was the last thing Goldman wrote for the screenplay.
The producers used the famous mansion of Marion Davies for the Sampson estate. They paid $7,000 for some refurbishments and gave Davies's widower Horace G. Brown a small, uncredited role as a bartender.
The 356 Speedster that Harper drives was one of only 140 made. A fully restored one has sold for $300,000 in auction.
The house used as the Sampson estate, Beverly House, is the same place used as Jack Woltz's mansion in The Godfather (1972).
Paul Newman: [H] The character Lew Harper is based on novelist Ross Macdonald's character Lew Archer. The name was changed for the film supposedly because Paul Newman had recently enjoyed success with Hud il selvaggio (1963) and Lo spaccone (1961) (two of his successful films beginning with the letter "H", a later one after Detective's Story (1966) was 1967's Hombre (1967)) and the producers wanted the movie's title to begin with "H". Also, the Macdonald estate did not want the name "Archer" used in the movie. There may have been fear of legal complications because Macdonald got the name "Archer" in the first place from Miles Archer, Sam Spade's partner who is killed early on in Dashiell Hammett's Il mistero del falco (1941).