Revealing mistake: When Azul is in the bar and goes into the men's room to execute the man inside, we look straight at the men's room door. You hear a gun shot, but you can see you can see a flash reflected. That means that what ever made the gunshot sound was behind the camera. Also in the same shot you can see a man's shadow on the door. (00:17:40)
Revealing mistake: When El Mariachi is trying to escape Moco's men by sliding down a wire onto the bus, he first throws the ball-and-chain-weapon over the wire. But then when he slides down, you can see a pulley device that he is using, not the chain. (00:26:45)
Answer: He was not the only professional actor in the film. However, the movie was made on a shoe-string budget, and, to save money, most of the cast were non-professional local residents.
raywest