When Louis (Robert De Niro) blows salt from his egg onto the restaurant table, Harry Angel (Mickey Rourke) takes a pinch and throws it over his left shoulder. The superstition is that by doing so you blind the devil (or your evil angel).
When Producer Alan Marshall first approached Robert De Niro by phone about taking a part in the film, De Niro asked him, "Are you the guy who produced Birdy (1984)?" When Marshall replied that he was, De Niro promptly hung up.
When Harry Angel (Mickey Rourke) visits Margaret Krusemark (Charlotte Rampling) for the first time, Margaret orders her maid to bring them tea. She and the maid speak briefly in French, which Angel obviously doesn't understand. The maid is asking "Should I bring out the best cups?" and Margaret responds "No".
A Fangoria article about the movie from the time when it was released, had a picture of the deleted death scene of Herman Winesap (Dann Florek). On one picture, Alan Parker is shown sitting next to the headless corpse with blood all over the room. The same article also shows picture of partially burned body of the journalist woman who slept with Harry (Mickey Rourke) earlier in the movie, laying next to the burned house.
"Girl Of My Dreams", which Louis Cyphre (Robert De Niro) plays at the end, is the same song that Epiphany Proudfoot (Lisa Bonet) sings in the bathtub earlier (and identifies as a Johnny Favorite tune that her mother always sang to her). It also has several appearances in the soundtrack: in Courtney Pine's saxophone solos, and played in a ghostly, one-fingered style on an out of tune piano. In one scene, when he is driving to the clinic where Johnny is registered as a patient, Harry Angel (Mickey Rourke) whistles along with the off-screen tune that he is not supposed to be knowing at that time.
Near the beginning of the movie, Harry Angel opens his bag and we see a set of skeleton keys. In the novel, Harry Angel uses a set of expensive skeleton keys to open locked doors. *It is also a reference to a "Hand of Glory" which Margret Krusmark mentions as well (which is the cut off hand from the corpse of a thief while hanging from the gallows and can be used to magically open all locked doors.)
According to United States Army records, Harry Angel's service number (O-168042) was assigned to Eugene M. Vigneron who was a World War II Coastal Artillery Colonel with follow-on Army Reserve service until retirement in 1962.
The train Harry Angel takes into Louisiana is known as the Southern Crescent. It was green in color, and marked by a Crescent moon. It ran from New York City to most of the major southern states and cities. Before Amtrak took over the line, the Southern Crescent was one of the last of the great American passenger trains.
A reference to the movie was included in Polish metal band Hunter's song "Armia Boga" (Pol, "God's Army") from the album "HellWood". The song ends with a phrase in English: "Welcome to my greedy Louis Cyphre's private hell."
In the film, private detective Harry Angel (Mickey Rourke) searches for someone called Johnny Favorite. Two years after this film debuted, O'Rourke starred as the title character in Johnny Handsome (1989).
"Johnny Favorite" is probably a name version for John the Apostle, one of the twelve Apostles of Jesus Christ. John the Apostle wrote the fourth Gospel, and is also the one who stood with Mary under Jesus' cross during the crucifixion. Hence "The Church Fathers consider him the same person as John the Evangelist, John of Patmos, and the Beloved Disciple" (1), or the "favorite". Since this movie is a story involving religion and the battle of good versus evil, Johnny Favorite may have been used as a Biblical name to add effect on the storyboard. However, there is a big difference between the Biblical Johnny Favorite, or John the Apostle, and this film's Johnny Favorite, as the latter is certainly one who did not associate generally with religion, and specifically with Christianity, but more so to black magic and Satanic deals to reach stardom. (1) - source, Wikipedia.
The trivia items below may give away important plot points.
Before each murder, Harry handles, or has something to do with the murder weapon, the Bible containing Fowler's gun, the dagger used on Margaret, Toots Suite's razor, the pot of gumbo, and his own gun.
The law firm Winesap and Mackintosh are two types of apples, traditional agents of the Devil tempting mankind. (Even though the fruit of the Tree of the Knowledge of Good and Evil is not identified as an apple in any Bible translation.)