Cast the Expert: The conductor on the subway train was the actual conductor, whose name was Bob Morrone. The actor who was supposed to play the conductor didn't show up on the day that scene was to be filmed. In addition, the motorman William Coke, was an actual N.Y.C.T.A. motorman. The Transit Authority refused to allow an actor to operate a subway train.
Irving Abrahams, who plays Irv the police mechanic, was the real-life NYPD mechanic who helped Eddie Egan and Sonny Grosso crack the "French Connection" case. As depicted in the film, Abrahams helped Egan and Grosso tear apart the car that French smugglers were using to sneak heroin into the U.S. When the movie was made, Sonny Grosso arranged with William Friedkin for Abrahams to play himself in the garage scene.
The real-life models of Doyle and Russo appear in the movie, Eddie Egan as the detectives' supervisor and Grosso as Klein, the BNDD Special Agent, Mulderig's partner.
Method Acting: Roy Scheider and Gene Hackman patrolled with Eddie Egan and Sonny Grosso for a month to get the feel of the characters. Hackman became disgusted at the sights he saw during this patrol. In one incident he had to help restrain a suspect in the squad car and later worried that he would be sued for impersonating a policeman.
No Budget: To save money on the budget and also because they didn't always have permits, William Friedkin had the cameraman carted around in a wheelchair instead of using a camera mounted on dolly tracks for the moving shots. This is most noticeable when Gene Hackman runs to then enters the subway car. As the camera follows Hackman hurrying towards the car the film movement is smooth but then shakes noticeably as the cameraman has to get up from the wheelchair and follow Hackman into the subway car.
Due to the low budget, no sets at all were built.
Shrug of God: The final scene of the film generated much praise and discussion for its ambiguity. In a BBC documentary, William Friedkin stated that the ending gunshot "doesn't mean anything-although it might."
In earlier interviews, Philip D'Antoni said the ending gunshot was put in to leave audiences with something to talk about as they left the theater, and Friedkin was of the opinion that Popeye had gone around the bend a little, and was shooting at shadows.
Throw It In!: Tradition has it that the famous baby carriage moment was an accident during filming. In truth it was staged. However, earlier in the chase scene there's a moment where Popeye smashes into another car while running the red light at Stillwell Ave. and 86th St., and that was a genuine accident that was left in. The driver was a random guy on his way to work. The production company paid for the repairs.
The traffic jam earlier on, while tailing Sal, was an accident. The actor playing Sal got too far ahead while the following car was caught in the traffic jam. It was also shot without permission.
Lee Marvin, rejected the film because he didn't like cops. He explained that he always made it a point to display some sort of conflict between his character and the military or the police, even though he would be a part of it. He felt that this was not possible with the film, and therefore could not get himself to accept the part.
William Friedkin had wanted an actor he had seen in the French film Belle de Jour to play Charnier because the actor was just like the rough-and-tumble gutter type the Real Life Charnier was. He couldn't remember the actor's name, but was told it was Fernando Rey. Rey was cast and flew to New York to meet Friedkin, only for Friedkin to quickly discover that Rey 1) wasn't in Belle de Jour, 2) was a suave, dapper gentleman who was nothing like Jean Jehan (the real Charnier), 3) was actually Spanish, and 4) didn't speak very good French. Friedkin discovered that the actor he'd actually wanted was a guy named Francisco Rabal, who like Rey was a Spanish actor who'd done some French films but didn't speak very good French...and, unlike Rey, spoke no English. So Friedkin reluctantly kept Rey, but later admitted that the suave/crude contrast between Rey and Gene Hackman suited the film very well.
Early in the film, Popeye hands Cloudy a straw hat to toss on the back deck of the car, saying it's time to "go to work". A straw hat, at least in the late '60s/early '70s, was a sign that an undercover detective was on duty.
For the "94% Pure" scene (where the heroin is tested), real heroin was used.