Brad Pitt Was Allegedly 'Volatile' On Set of 'Legends of the Fall'

Brad Pitt was a young and restless actor on the set of "Legends of the Fall" way back in the '90s -- so claims the director of that film ... who's painting BP as passionate to a fault.

Edward Zwick -- who helmed the famed romance epic -- which launched Brad into orbit as a heartthrob hunk and a mainstream A-list actor -- has this new book coming out called "Hits, Flops, and Other Illusions: My Fortysomething Years in Hollywood."

Vanity Fair obtained some excerpts from the forthcoming memoir ... and there's a fair amount of writing about Brad, who was the lead star of the movie as one of the Ludlow brothers, the free-spirited and wild-hearted Tristan.

As it turns out ... the character and the man playing him might've not been so different -- at least according to Zwick, who says Brad could be "volatile when riled" while they were shooting. Even from the jump EZ claims Brad wanted to quit after just the first table read.

Once they were able to get Brad to stay on, they would often clash when it came to getting the best takes out of Brad -- not to mention what each of them wanted from the character ... especially when it came to showing different sides of himself as an actor.

He writes that Brad would "get edgy whenever he was about to shoot a scene that required him to display deep emotion." Zwick says he wanted Brad to express himself more in portraying Tristan -- but he says Brad would challenge him and push back.

Zwick then says he once gave an acting instruction to Brad in front of the crew, and that they got into a heated argument ... which Zwick describes this way, "I don’t know who yelled first, who swore, or who threw the first chair. Me, maybe? But when we looked up, the crew had disappeared. And this wasn’t the last time it happened. Eventually, the crew grew accustomed to our dustups and would walk away and let us have it out."

In the end, they were able to work past their creative issues and finish the film -- with Zwick ending on a positive note about Brad ... calling him "forthright, straightforward" and "capable of great joy." Frankly, that's exactly what Tristan Ludlow himself was like!

It's also pretty funny that Zwick feels like Brad wasn't emoting enough on camera -- fact is, if you've seen this movie, you know Brad goes balls to the wall as a thespian ... and there are a lot of tears shed by his character.

The way Zwick characterizes things now -- makes you wonder if we only saw the raw emotion Brad showed on the big screen because the director truly pushed him to let it out.