In the upcoming drama, the Gladiator star plays Freddie Quell, a troubled World War II veteran who turns violent when he begins questioning his mentor Lancaster Dodd, portrayed by Philip Seymour Hoffman.
And, in order to get in the right mindset for a scene in which the pair get thrown into neighbouring prison cells, the 37 year old watched videos of animals held in captivity in a bid to imitate their deranged habits.
He tells the New York Times, "You can see their brains don't seem to be functioning anymore. It's pure reaction, and you see the muscles going. I knew that's what I wanted to capture - they're hurting themselves, and they don't even know that, but something inside is saying get out, get out."
And Phoenix got so caught up in the shoot, he actually smashed the cell's toilet in the midst of a head-banging rage: "I didn't intend to break the thing. I didn't know that was possible."