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 survival thriller about Nick and Lori who eloped to get married in Las Vegas then drove to the Grand Canyon hoping to take a mule trip to the bottom. They can't get a permit so they hire a shady guide they meet in a bar named Henry. He brags to them about surviving attacks by grizzly bears, mountain lions, bobcats, mules and a rock slide, and shows them his scars as proof. But after all that, he doesn't survive the rattlesnakes in this movie.
On their first day, they get to the bottom of the canyon where Henry convinces them to ride a few more hours off the official trail to see some secret petroglyphs. Just as Henry is taking a drink of alcohol a rattlesnake strikes at his mule. The mule throws him off to the ground where he ends up next to another rattlesnake that strikes at him and bites him on the arm. He pull his knife and cuts at the snake throwing its body a few feet away, but then a second snake strikes and bites him on the face. He takes his knife and cuts the head off the snake, leaving it hanging from his cheek by its fangs. He pulls out the fangs and throws the head away and sits on the ground with the still-moving body of the headless snake draped over his legs. Nick kicks the snake's body off him and Henry tells him his arm is broken and he's snakebit twice. He tells Nick the snakes were diamondback rattlers and he's lucky they weren't Mojave Rattlers because their venom goes straight to the spinal cord and that's not a pleasant way to go. (The spinal cord part is not true.) He tells Lori to put a splint on his arm to immobilize it and keep it below his heart and that cutting the wounds invite infection. (At least that part is good advice.)
This leaves them in a bad situation - the mule with their supplies ran off so they have no food little water and no cell phone reception and they're miles from where anybody would be looking for them. Henry suggests they eat a rattlesnake and tells Nick to cut the connecting muscles around the anal vent to release the skin. Nick cuts it and cooks it and tells Lori it tastes like chicken, but she wont eat the snake because she became a vegetarian as soon as she found out the snake had an anal vent.
The next day Henry wakes up half crazy with a swolen face and then dies. The rest of the movie is the survival thriller part of the movie where Lori has to fight off wolves and cut off Nick's leg with Henry's knife. It's not very realistic, but it's insane enough to keep your attention until the ironic tragic ending.
Sure, this is just another example of using a rattlesnake bite to get rid of a character that is no longer needed, but it's not an entirely impossible situation, just extremely unlikely. (And no rattlesnake can ever stretch as far as we see here when it strikes at the mule.) The nerd in me insists on telling you that there are no Western Diamond-backed Rattlesnakes in the Grand Canyon, and probably no Mojave Rattlesnakes either. Nevertheless, the live snake we see is a Western Diamondback, so I'm giving Henry credit for the correct ID, even if some of his information is wrong. We also see several artificial snakes used to represent the striking snakes, the snake head, and the dead snake. The dead snake that Nick holds to cut up is a real dead snake but it's a different snake from the diamondback that it is supposed to be, probably a Western Rattlesnake. Using all these different-looking makes sense in filmmaking, and it's probably not obvious to the casual viewer that the snakes keep changing appearance, but I'm an annoyingly uncasual viewer when it comes to snake scenes so I was laughing, especially at the severed rattlesnake head still attached to Henry's face. That was a nice touch.