What exactly happened to Aaron Eckhart's character?

From the novel, The Last Ride, it wasn't Brake who was killed this way, but the killing was in it. From page 67:

He took a deep breath and then held it and slit the stitching of green thongs; the cowhide flaps spread wide, releasing a cloud of putrid-smelling steam, and Jones gagged on it, turning away. Death, in general, had never bothered him much. But this one did.

The body was curled in a tight ball and disfigured to the point it was almost unrecognizable. Nevertheless, he knew it wasn't Lily. He picked up one side of the cowhide and rolled the corpse onto its back. Mannito. The little Mexican was naked and covered with thousands of tiny puncture holes; but those had not killed him. His death had come from the green cowhide. The fire was slowly drying it, shrinking it, until it finally crushed his ribs and suffocated him.

...

Most likely they'd ambushed him and Lily, shooting the rancher and grabbing the girl, and then, later, been surprised by Mannito. Somehow they'd caught the little Mexican alive, stripped him naked, put a rope around his chest and dragged him back and forth through the prickly pear. Afterward, with a thousand cactus thorns impaling him, they'd beaten him with clubs and then sewn him up in the hide and hung him from the oak like a giant cocoon.

In the Novel, Brake was simply shot. Another character, Mannito, comes upon the girls and their captives and is caught and tortured. When Ken Kaufman turned Thomas Eidson's book into a screenplay, it was decided that the character of Mannito was to be removed, and what happened to Mannito was instead shown as happening to the character of Brake.

The Indians used un-cured cowhide (which was 'fresh' from the animal, versus having been stretched and cured into usable fabric), sewed Mannito/Brake into it, and hung him from an oak tree over an open fire. The heat of the fire caused the hide to shrink, eventually suffocating him.

This wasn't a 'ritualistic' killing in the sense that they did it for 'Power'; it was a means to ensure that a living victim died in as long and painful a way as possible, without them having to actually stay until the (inevitable) end. Dragging the man over/through the prickly pear ensured he was too wounded to have a chance of fighting his way out of the trap.

He was packed inside deer hides and hung over an open fire to bake...like being put into an oven...

It basically crushed him. The fresh cowhide would have shrunk in the same way headhunters shrink heads by pulling out the skull and roasting it. If the smoke or heat had have killed him he wouldn't have been screaming long.

I believe he was beating n bled leaving him defenseless and the poison with the green stuff which made his centrak nervous system extremely sensitive and the tied his feet to his hands and droped him in cow hide sewn it shut and had him hanging right above fire which burned with coal reaching the extremely high temperatures without direct flame. Contact Making extremely high temperatures cure the hide as well as BBQ Echart at high temperatures his blood, and organs would slowly swell as the blood boiled and organs and tissue swelled do to extreme heat and at same time cow hide got tighter and tighter snapping his spinal column as his back vertebra beging popping and dislocating like rest of joints and the the heart valves and arteries would ruptute and last 2 mins of his life hot steaming blood and other organ fluids steam out his mouth and feel 10x more painful thanks to pain administered before hand.