The character Nevada Smith was supposed to be 16. He was played by 35-year-old. His age of 16 is never mentioned however Steve McQueen.
In the scene in the cattle pens when Max fights Jesse Coe, Max crouches behind a fence and opens the gate to let the cattle out. Some cattle come out the gate while others knock down the fence, and Max must dodge the flailing legs and hooves of the stampeding cattle. The knocking down of the fence was accidental, and Steve McQueen was very nearly trampled for real. Shots of Max rolling clear of the hooves were added when it was decided to use the accidental footage.
Suzanne Pleshette once said in an interview that her love scenes with Steve McQueen in this movie were horribly awkward for both of them, as they had enjoyed a completely platonic friendship since she first came to Hollywood and he had very much taken on the role of big brother to her.
Steve McQueen loved to play with props as it kept the audience's eye on him. When Jonas gives him a silver dollar, McQueen slaps it back and forth in his hands. This upstaging by playing with props would often annoy other actors. Yul Brynner, who played his friend in Les 7 mercenaires (1960), found this very annoying, as McQueen would play with his hat, his gun, check his bullets, etc., constantly drawing attention to himself.