The blonde wig that Barbara Stanwyck is wearing throughout the movie was the idea of Billy Wilder. A month into shooting Wilder suddenly realized how bad it looked, but by then it was too late to re-shoot the earlier scenes. To rationalize this mistake, in later interviews Wilder claimed that the bad-looking wig was intentional.
The character Walter Neff was originally named Walter Ness, but director/writer Billy Wilder found out that there was a man living in Beverly Hills named Walter Ness who was actually an insurance salesman. To avoid being sued for defamation of character, they changed the name. In the novel, his name is Walter Huff, and Dietrichson is Nirdlinger
The movie was based on the novel by James M. Cain, which in turn was based on the true story of Ruth Snyder, the subject of a notorious 1920s murder trial.
The part of Walter Neff was originally offered to George Raft. He insisted that he would only take on the role if his character turned out to be an FBI agent at the end, entrapping Barbara Stanwyck's character. As this ran completely counter to James M. Cain's original novel, he naturally didn't get the part.
The scene where Neff and Dietrichson can't get their car started after the murder was added by Wilder after his car wouldn't start at the end of a shooting day.
The victim, Mr. Dietrichson, is an oil company executive. Screenplay writer Raymond Chandler was an oil company executive before he became a writer.
Various studios expressed interest in the story when it first appeared in serial form in 1935 but realized it was unfilmable within the strictures of the newly-established Production Code.
We never learn the first name of Mr. Dietrichson.