Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a western adventure directed by Howard Hawks filmed in Grand Teton National Park. It's about the first men to take a keelboat up the unexplored Missouri River through hostile Indian territory to return an Indian Princess to her tribe, the Blackfoot Indians, and open up a fur trade with them. It was nominated for a best black-and-white cinematography Oscar, but I saw the colorized version, which was a bit disappointing. Still, it was worth it for the scenery and to see a young Kirk Douglas and a great Oscar-nominated supporting role by Arthur Hunnicutt.
The snake appears in the very first scene. Somewhere in Kentucky in 1832, Jim Deakins (Kirk Douglas) is driving his wagon when he thinks he sees something moving in the woods. He gets out of the wagon to investigate, but he steps on an unstable tree branch and falls off and lands on the ground next to a tree with a rattlesnake lying at its base. He's startled by the snake and falls back, then pulls out his knife, but suddenly a strange man appears and throws a knife at the snake. Deakins thanks him and returns his knife. His name is Boone Caudill (Dewey Martin) and Deakins invites him to ride in his wagon. This is the first meeting of the two men, who eventually travel together to St. Louis, where they get on the boat and journey up the Missouri River.
The snake is a live rattlesnake, but it's hard to tell what kind because of the phony color and because we can't see it clearly. We know it's a rattlesnake because we hear rattling sounds and we (barely) see a rattle on a tail. The rattle doesn't move so it might be a prop stuck in place to look like a part of the snake. You can also see a string tied to the snake's neck to lift its head up, and maybe to restrain it. The knife Boone throws does not hit the snake, it hits just below it, but I think we're supposed to believe that Boone killed it with the knife, even though the snake is still moving around after the knife hits.
Much later during the journey, Boone is wounded by a knife by the Indian Princess. (It seems stabbing is her way of showing affection.) While trying to thread a needle with deer sinew to sew the wound, Boone's uncle, Zeb Calloway, (Arthur Hunnicutt) tells Deakins a story about sewing a man's severed ear back on backwards and how that interfered with his perception of rattling rattlesnakes:
"I remember once me and a trapper named Parker - we run smack into a big grizzly bear. Bear sure made a mess out of Parker before we killed it. Ripped one of his ears clean off. But this child just happened to have a needle and some of this here deer sinew just like we got here. Yeah, while his ear was still hot, I picked it up and sewed it back on his head, and it growed almost as good as ever.... I said growed almost as good as ever, not hardly. Seems I sewed Parker's ear on backwards. Yeah, he hated me till the day he died... on account of every time he'd hear a rattlesnake, he'd turn the wrong direction and step smack into it."