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The plot is inspired by true events, but often barely follows them. Hugh Glass was attacked by a grizzly in August 1823 in the Grand River area in north-northwest South Dakota, near the present town of Shadehill, south of Lemmon. There's even a Hugh Glass Recreation Area there. The area is slightly hilly, Great Plains prairie. That region is 2000-2500 feet above sea level, and the average temperatures in August vary from 55-100+ degrees. The closest place with anything resembling a mountain is an overland direct-line trek of 130-150 miles, in the Black Hills. August temperatures there range from 55-85 degrees, and at 7000-8000 feet the Black Hills are nothing like the mountains in the movie. The nearest mountains remotely like the movie's are the Bighorns in Wyoming, which are almost 300 miles away and still have August temperatures in the 55-85 degree range. At the time Glass was attacked there wasn't any snow, no one was close to freezing and no one was anywhere near terrain like that shown in the film. His trek ended at Fort Kiowa near Chamberlain, SD on the Missouri River, about 200 miles southeast of the attack site.
Director Alejandro G. Iñárritu originally wanted a huge pile of human skulls to be showcased during one of the dream sequences. He intended for this to symbolize the future demise of the Native Americans in the film. Production designer Jack Fisk then showed Inarritu a photo from 1850 of men standing among a huge pile buffalo skulls and explained to him about the awful history of the Native Americans being deprived of their food source by the government. Inarritu decided to change the scene from human skulls to buffalo skulls in order to recreate the photo.
Shot chronologically on an 80-day schedule that took place over a total principal photography time period of nine months. This unusually long production time was due to the cold weather conditions, the remoteness of the locations and director Alejandro G. Iñárritu's and cinematographer Emmanuel Lubezki's aesthetic plan to shoot only with natural light for maximum realism. Only a few shooting hours were available every day and had to be carefully planned in advance.
Due to production being behind schedule, the snow melted during the location shoot in Canada before filming was complete. With summer rapidly approaching, there was no choice but to relocate the entire production to southern Argentina, where there were similar wintry conditions.
The film was shot only with natural light but with only one exception, according to cinematographer Emmanuel Lubezki: a campfire sequence at night where the wind was causing the fire to pulse in a distracting way. "We had to lay a bunch of light bulbs around the fire to create a cushion of light," Lubezki admits. "That's all the light we used." [16 Dec. 2015, Variety]
Leonardo DiCaprio chose to devour a raw slab of bison's liver, even though he is vegetarian. He also had to learn to shoot a musket, build a fire, speak two Native American languages (Pawnee and Arikara), and study with a doctor who specializes in ancient healing techniques. DiCaprio calls it the hardest performance of his career.
Leonardo DiCaprio: [Immersed in water] In this film, Hugh Glass (DiCaprio) is shown in several scenes involving water. In a long sequence, Glass is swimming in the rocky river to get away from danger and start his journey. The scene before he comes across a herd of bison, he's standing in a river catching a fish. And much later, after being rescued and brought to Fort Kiowa, there's a brief scene where he's bathing as he pours water all over himself. See also Romeo + Juliet (1996), Titanic (1997), The Beach (2000), Inception (2010), Shutter Island (2010), The Great Gatsby (2013), The Wolf of Wall Street (2013), and Once Upon a Time... in Hollywood (2019).