To depict mountain men in the uncharted frontier of the 1800s US for his new movie "The Revenant," director Alejandro González Iñárritu ("Birdman") was obsessed with making every aspect of the film as authentic as possible.

For the actors, that involved going through a one-week mountain-man boot camp. The production hired a man named Clay Landry to head it. Landry works as a historical consultant at the Museum of the Mountain Man in Pinedale, Wyoming.

In late September 2014, Landry worked with the actors on everything from how to shoot a black-powder rifle to how to start a fire with only flint.

"Alejandro told me, 'I want these guys to look like they have been handling these guns all their lives,'" Landry told Business Insider.

To accomplish that, Landry didn't just have Leonardo DiCaprio, Tom Hardy, and the other cast do target practice all day. He also made them go by foot on trips along the river and hills in Canada where part of the film was shot, and he gave them some history lessons.

"I would do a little bit of a lecture at the start of each day," Landry said. "Because the first thing we realized was most of these actors were from places other than the United States of America, so we had to give them some context of what the whole era was about and why these men were in the mountains."

Landry said that the actors took to the boot camp quickly. They would even compete in tomahawk-throwing contests when they broke for lunch.

But Landry's job wasn't done after the weeklong boot camp. During filming, he was on set answering any questions the cast and crew had. Landry recalls that because it rained often, he had to correctly show how the rifles would be covered in those conditions.

"We went over this in the boot camp, and I was happy to see the actors remembered when shooting, but in rainy conditions back then you always had to cover your rifle, especially the action part of the gun," Landry said. "Because any moisture gets into that black power, it won't ignite. So we had the right stuff there on-screen because a mountain man walking around in a rainstorm without his gun covered would just not happen."

But sadly, you won't see Landry's biggest highlight in "The Revenant." During the Indian attack scene that opens the film, Iñárritu wanted extras who were skinning beaver to talk like they would in the 1800s, though their dialogue and even some of their faces did not make the final cut.

"So I actually sat down with one of the second assistant directors and we wrote a script for the extras so they would be talking about correct stuff of that period while they were shooting," Landry said.

What were the lines they came up with?

"Stuff like 'What do you think the price of beaver in St. Louis is now?' or 'Did you see that girl back in town? She was really something' — that kind of stuff," Landry said with a laugh.

"The Revenant" opens in limited release on Christmas Day and everywhere January 8.

SEE ALSO: Leonardo DiCaprio's new movie is a brilliant, gory revenge tale that could win him his first Oscar

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