Eddie Egan, the real police detective who inspired the character Popeye Doyle, plays Doyle's superior in the movie.

The casting of Fernando Rey was an accident. William Friedkin had wanted to cast Francisco Rabal, whom he had seen in Belle de jour (1967), but at the time didn't know his name. Only after actor and director met did Friedkin realize Rey wasn't the actor he had wanted. Friedkin ended up keeping Rey after learning that Rabal spoke neither English nor French.

During the subway chase scene Hackman collides with another vehicle. This wasn't supposed to happen and the vehicle wasn't even supposed to be on the set. The crash looked cool, so it was kept.