"He's definitely going to make waves with his performance because people are curious," he said. "It just depends on how people accept him."
That includes his shaggy new look. "I was surprised by his appearance," Snedegar said, referring to Phoenix's bushy beard, dark shades, baggy sweater and torn gray knit cap. "Everybody was. He was dressed very laid back, a beachy vibe with loose pants, a big sweater and a cap on most of the time. He looked like he had gained a little weight."
Phoenix's dramatically different appearance, coupled with reports of erratic behavior, have some worried that the former leading man is headed for the same self-destructive path as his older brother, the late actor River Phoenix.
"Given his actions, he [Joaquin] is starting to look a lot like his brother," Ian Drew, editor at large at US Weekly, told ABCNews.com. "It does make you question how his story is going to end -- or continue."
River Phoenix died of a drug overdose in October 1993 after collapsing outside the Johnny Depp-owned Viper Room nightclub in Los Angeles.
The then-19-year-old Joaquin, who was at the club along with their sister Summer, made the 911 call to try to save his 23-year-old brother's life.
"Joaquin is definitely on a course right now," E! Online gossip columnist Ted Casablanca told ABCNews.com. "I don't know if it's self-destruction or self-derailing. But he's obviously going through some sort of tortured episode in his life. It's a cry for help."
"He's obviously a deep guy with some dark reference points," Drew said. "There's obviously a shadow over his life. He's had problems played out on the public stage."
In 2005, after filming the Cash biopic "Walk the Line," Phoenix entered rehab for alcohol abuse. He told reporters afterward that he started drinking more while playing Cash.
"It was then that I became aware of my drinking. I wasn't an everyday drinker but didn't have anything else to do, anything to hold me down," he said. "I was leaning on alcohol to make me feel OK. That's what it really was."
Publicist Patricola quashed any rumors that Phoenix is self-destructing.
She told ABCNews.com, "He is fine and doing his music," but would not go into detail. "When Joaquin has more to say, I'm sure he will," she said.
Snedegar said Phoenix appeared fine the night he performed. "Every time I spoke to him he was coherent, polite. He was low key but open to the fans."
Even though Phoenix's agent had booked the gig a couple weeks before, it was still undecided whether he would perform that night. The evening started with a specially prepared vegan dinner for Phoenix and 20 friends, including Affleck, at the Palazzo Resort Hotel and Casino.
The group then moved upstairs to Lavo, Snedegar says, where they sat at a table in front of a catwalk with a microphone, in case Phoenix decided to perform.
A member of Affleck's documentary crew gave the signal: Phoenix was ready to go on.
"I don't know if I would say he's a natural," Snedegar added. "He isn't horrible. He can keep on beat. He was kind of dancing and on rhythm and that was good."
Phoenix, who seemed tentative at first, Snedegar said, soon warmed up during the course of the three raps, strutting the catwalk and, by the end, jumping up and down and thanking his audience profusely. "You hold a very special place in my heart," he told the crowd.