Whatever their process was, it seems to have worked, as the film has been nominated for a total of twelve Academy Awards, including Best Picture and Best Director, in addition to Best Actor and Best Cinematography. In this NYT Anatomy of a Scene video, director Alejandro González Iñárritu elaborates on the difficulty of a particular escape scene, which required actor Leonardo DiCaprio and cinematographer Emmanuel Lubezki to enter freezing waters:

And here's the trailer if you haven't seen the film yet:

Here's more from the NYT interview with Iñárritu:

Your cinematographer, Emmanuel Lubezki, went into the water to shoot this?

Yes. He went in with the camera and to do these choreographed movements. Every movement was designed months in advance. The second part of the river sequence, in Montana, was shot 11 months after the first. It took us almost a year for us to understand how we were going to do it technically.

The first part of the sequence, which was shot in Calgary, required DiCaprio to wear special clothing under the pelt so that he could stay in the water for longer periods. Adding to the difficulty was the fact that the pelt got much heavier when it was wet, so swimming became even harder. The important part of all of this is that they choreographed the scene as much as they could. There is a certain amount of risk involved in filming dangerous scenes, but that risk should be minimized as much as possible for everyone involved. 

While they didn't fake anything in The Revenant, it's hard not to wonder how much of that suffering shows up on screen. Would it have been a different film if it wasn't quite so cold and they didn't shoot for quite so long? It's impossible to know, but certainly there were more than a few people who believed in this process and came along for the ride, and it's already paid off in awards season. 

Source: New York Times