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.
Peter Boyle was offered the role of Popeye, but declined after having starred in Joe, a film about a bigoted factory worker who eventually murders a bunch of hippies, which had audiences cheering at him instead of being revolted, much to Boyle's disgust.
Steve McQueen was offered the role of Popeye, but declined because he didn't want to play another cop after Bullitt.
Jackie Gleason was also considered for the role, but finally rejected by 20th Century Fox due to his box office failure in the studio's 1962 film Gigot.
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.