Last Thursday in Los Angeles, a crowd of low-key Hollywood types — actors, producers, Moby — crowded into the Taschen Gallery for a party to celebrate the new film “You Were Never Really Here.”

Meanwhile, the movie’s star lurked outside, utterly unnoticed. When he finally came in, sporting a grizzled beard and silver-tinged man bun, Joaquin Phoenix retreated to a shadowy corner with actor pal John C. Reilly and two other scruffy dudes.

“He’s really outside his comfort zone tonight,” observed a longtime Phoenix friend. “And his comfort zone doesn’t go too far to begin with.”

In fact, when Phoenix, 43, was named best actor for the thriller at Cannes, he leaned over to actress girlfriend Rooney Mara to ask her what had just happened. Stumbling to the stage, he looked more traumatized than triumphant, barely managing to stammer out a few self-deprecating words.

Although he’s been nominated for three Oscars over the years, Phoenix has always seemed adrift — even miserable — in the trappings of Hollywood. (He once called the awards “total, utter bulls – – t.”) And some are saying Hollywood’s most tortured star has a year of torment on the horizon.

Besides “You Were Never Really Here,” Phoenix has three other films slated: playing a paraplegic in “Don’t Worry, He Won’t Get Far on Foot,” a traumatized assassin in “The Sisters Brothers” and Jesus Christ in “Mary Magdalene” (which co-stars Mara and is looking for a new distributor since the Weinstein Company went bankrupt).

“The reason he’s good at playing traumatized is that he was traumatized,” said a film producer who’s worked with the actor. “He grew up . . . in a cult. His brother died in front of him. None of that presupposes someone who’s going to turn out normal.”

Born Joaquin Bottom to hippie parents in Puerto Rico, he spent his early years in the Children of God cult (which at one time promoted sex with minors) and performed on the street for tips. Once they left the group and moved to Los Angeles, he and his four siblings began acting. Oldest brother River got famous first. It was Joaquin, then 19, who made the 911 call when River overdosed in front of the Viper Room in 1993.

Phoenix got his big break when director Gus Van Sant, a pal of his brother’s, cast him in 1995’s “To Die For.” Since then, his roles have included a sadistic emperor (“Gladiator”), a drunken sex addict (“The Master”), a stoner detective (“Inherent Vice”) and a demon-plagued Johnny Cash (“Walk the Line”). Insiders say he chooses such parts for the chance to be someone else.

“He’s better at being a character than being himself,” said the producer. “He really thinks he’s boring.”

Phoenix took it too far in 2010 with the faux documentary “I’m Still Here,” for which he spent a year living as a reluctant movie star giving it all up to be a rapper. On a now-infamous 2009 appearance on “Late Show With David Letterman,” he did little more than mutter and nod out, leading Letterman to compare him to the Unabomber. When he returned to the show the following year, Phoenix explained it was all a hoax, a way of disassembling his own fame.

“Hiding is just a natural state for him,” added the producer.

Phoenix eschews LA’s favored lifestyle: He reportedly doesn’t hike, go to the beach or hit major restaurants — preferring a strict vegan diet of vegetables from his own garden.

“His circle of friends is as tight as he can keep it,” adds the friend. “Not a lot of people are going to get him . . . [and] he’s good with that.”

Sources point out that Phoenix became more of a recluse after a stint in alcohol rehab in 2005. And he’s found the perfect introvert partner. After only a few relationships, including a romance with Liv Tyler in the ’90s and seeing a 19-year-old DJ when he was 39, Phoenix is now with wallflower Mara, 32.

“They’re meant for each other,” said the friend. “Both [are] shy homebodies, both loathe press.” When the two have ventured out, they’ve been spotted at vegan restaurants and a colonics spa.

“God forbid he gets an Oscar this year,” said a publicity exec who has worked with Phoenix. “He may not make it to the stage. Everyone wonders, ‘Is he really that sensitive — or does he just want you to think he’s weird?’ You’ll always be confused about him. Which actually is kind of weird.”