VENICE - The last time Joaquin Phoenix was at the Venice Film Festival, in 2010, he was many months into a supposed career transition - from acting to rap - that coincided with obvious weight gain, an unruly new beard and several widely noted instances of erratic public behavior. Much of this was chronicled in "I'm Still Here," a supposed documentary by Mr. Phoenix's friend Casey Affleck, and all of it was later revealed as a satirical performance-art project.

Mr. Phoenix, 37, returned this year, and things were more auspicious. He shared the festival's best actor prize for its most anticipated movie, Paul Thomas Anderson's "The Master," with his co-star, Philip Seymour Hoffman. Mr. Anderson won for best director, which all bodes well as awards season gets under way.

Before its premiere on September 1 there had been speculation that "The Master," Mr. Anderson's first film since "There Will Be Blood" in 2007, would be an expose of Scientology. But its story of a new fringe religion in postwar America, while inspired by Dianetics and L. Ron Hubbard, is merely the backdrop for an exploration of man's animal nature and civilization and its discontents. This is refracted through the relationship between a seductive guru (Mr. Hoffman) and a follower with a wild streak, played by Mr. Phoenix in his first screen appearance in two years.

Mr. Phoenix said that his last film - although greeted in most quarters with befuddlement or hostility - represented an important turning point. "It completely broadened my perspective on acting," he said. "I wanted to have that same experience that anything is possible."

Looking to re-establish himself, Mr. Phoenix read many scripts. Freddie Quell in "The Master" was the first role that drew his interest. A troubled World War II veteran, Freddie is both the feral opposite number of and the unlikely kindred spirit to Mr. Hoffman's smooth-talking peddler of salvation, Lancaster Dodd.

Mr. Anderson said he wrote the role of Dodd for Mr. Hoffman, a regular collaborator; for Freddie, he needed "a formidable opponent for Phil."

The prospect of working with Mr. Phoenix excited them both: "I remember Phil saying, 'Joaquin scares me, in a good way,'" Mr. Anderson said.

Mr. Phoenix started as a child actor, and since his breakthrough, at 21, in Gus Van Sant's "To Die For," he has emerged from the shadow of his late brother, River. He has two Academy Award nominations, for playing the villain in "Gladiator" and for his portrayal of Johnny Cash in "Walk the Line." But in none of his previous roles does he approach the sheer volatility and physicality that he brings to "The Master."

"I knew he was going to be good, but I didn't know he was going to do this," Mr. Anderson said.

"I'm Still Here" inspired Mr. Phoenix to experiment. "Going out on a stage publicly and not knowing how people are going to react to you - once I experienced that, it made me feel much more comfortable," he said.

His reliance on instinct meant that the first takes were often the most electrifying. "He's pretty bad at faking it," Mr. Anderson said. "If a moment gets lost, it's pretty hard for him to come back to shore. It became clear to me that you better get your lighting right the first time."

Mr. Phoenix spoke of his rejuvenated career - next up: films with James Gray and Spike Jonze - as a renewed appetite for risk. "For some people acting is a sunset stroll on a beach, and for others it's scaling a cliff or jumping out of a plane," he said. "In so much of my life, I take sunset strolls on the beach. When I act, I like the idea of jumping out of the plane."

The New York Times