Actor-Inspired Element: Clint Eastwood helped in creating his character's distinctive visual style. He bought the black jeans from a sport shop on Hollywood Boulevard, the hat came from a Santa Monica wardrobe firm and the trademark black cigars came from a Beverly Hills store. Eastwood himself cut the cigars into three pieces to make them shorter. Eastwood himself is a non-smoker.
Beam Me Up, Scotty!: It's "You see, my mule don't like people laughing," not "My mule doesn't like to be laughed at."
Executive Meddling: A ten-minute prologue (hastily filmed by the studio and featuring a body double for Clint Eastwood whose back is facing the camera) was attached to the film when it was first aired on network TV. Why? The guy in charge of ABC HATED the film's amoral plot and refused to air it unless the studio "fixed" what he saw was the impure motive of Joe. Hence the prologue, which rewrites the entire plot via a single ten minute scene where it is "revealed" that Joe was a former convict who was given his freedom in exchange for doing the bidding of the warden of his jail, in terms of driving out the gangs from the town in which the film takes place. Thankfully, the scene only was used for one airing and was promptly destroyed by the studio after the fact... although it has somehow made it onto the DVD as a special feature.
Follow the Leader: Everyone and their dog knows this film is an unauthorized remake of Akira Kurosawa's Yojimbo. For posterity, Kurosawa is on record as enjoying the film, calling it "a fine movie, but [...] MY movie," Leone had been assured by his production company that legal issues over the similarity had been ironed out before shooting, and eventually a legal settlement was reached.
Hostility on the Set: Gian Maria Volontè reportedly didn't get along with Sergio Leone, who found Volonté's theatrical acting style and arrogant on-set manner tiresome. Volonté tried to become friendly with Clint Eastwood, but the language barrier and political differences (Eastwood was a conservative Republican, while Volonté was a committed leftist) prevented their striking up a rapport.
Late Export For You: The fact that it cribbed Yojimbo's plot almost verbatim meant that Akira Kurosawa was able to successfully sue for copyright infringement. As a result, Kurosawa's production company obtained exclusive rights to release the film on Japan and the film took a whopping three years for its release to be allowed on the United States.
Money, Dear Boy: Gian Maria Volonte didn't think much of this film (or its sequel), telling interviewers that he considered them paycheck roles between his more personal projects. Additionally, Eastwood and Leone both claimed that Volonte was difficult to work with, arguing with his costars about Italian politics and, on one occasion, storming off the set and driving to a hotel until Leone calmed him down.
Prop Recycling: Joe's famous snake handled pistol was first used in Rawhide, which Rowdy Yates took from an outlaw he killed. He also wore the same boots from the series.
Troubled Production: Mostly due to Leone's fractious relationship with Jolly Films, who gave Fistful a miniscule budget, assured Leone that legal issues over the Yojimbo similarities had been cleared before shooting started (they hadn't, resulting in a long, acrimonious lawsuit) and fumbled its initial release, dumping it into second-run theaters and as the second feature on double bills. Eventually the movie became a hit despite its shabby treatment, allowing Leone to make For a Few Dollars More without Jolly's help. That film's title was explicitly a Take That! directed at Jolly Films.