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 of sorts.

Mr. Phoenix, 37, is back here this year, and things are a bit more straightforward this time. He’s the star of the festival’s most eagerly anticipated movie, “The Master,” directed by Paul Thomas Anderson, and to judge from the response from critics and audiences here, a leading contender for its best actor prize, which can only bode well as awards season gets under way.

Before its premiere on Saturday there had been speculation that “The Master,” Mr. Anderson’s first film since “There Will Be Blood” in 2007, would be an exposé of Scientology. But its story of a new fringe religion in postwar America, while inspired by Dianetics and the early career of L. Ron Hubbard, is merely the backdrop for an exploration of man’s animal nature and civilization and its discontents. These issues are refracted through the relationship between a seductive guru (Philip Seymour Hoffman) and a follower with a wild streak, played by Mr. Phoenix in his first screen appearance in two years since “I’m Still Here.”