This is one of only two films which Bruce Lee acted in where he spoke with his own voice (the other being Enter the Dragon (1973)). This is also the only film in which Lee played a villain.
In the nightclub, Marlowe (James Garner) takes a sip of wine and, smirking, judges it to be "impertinent... even baroque." These were the exact words which a character in Gore Vidal's Myra Breckinridge (published a year earlier) had used to describe Garner's butt in an excerpt from an obtuse film journal which appeared in the novel. Obviously, this was an inside joke and from Garner's smarmy delivery of what was otherwise a pointless remark, he was very much in on the gag.
The phone number seen in the opening scene in the hotel, 555-2368, is the same number seen on Jim Rockford's phone in the opening credits off The Rockford Files (1974).
While struggling to find acting work in Hollywood, Bruce Lee had a side career instructing Hollywood figures in martial arts. Amongst those were this film's writer Stirling Silliphant, and lead actor James Garner.
The Little Sister, the source novel for this film was set in 1940s LA. The film retains the book's setting, but was updated to the 1960s. The book was the fifth Phillip Marlowe novel written Raymond Chandler. Screenwriter Stirling Silliphant said he had to create "90% of the dialogue" because he felt Chandler's original was dated.