10-08) 19:59 PDT -- In "La Mission," an upcoming movie set in San Francisco's Mission District, Christopher Borgzinner plays a street thug who clashes with the main character, an ex-convict-turned-bus driver played by Benjamin Bratt.

The 18-year-old Borgzinner was never in a gang as he grew up in the city's Portola neighborhood, but drew from what he saw going on around him and his own experiences as a young Latino to inspire his acting in the film.

On Monday, real-life violence descended on Borgzinner as he rode a packed 9-San Bruno Muni bus to his acting class in the Mission, wearing the wrong-color shoes.

Wrong because they were red, the color claimed by the Norteño gang. A group of men, ignoring his denial of any gang affiliation, stole his wallet and other items and put him in the hospital with a beating.

"It was just a senseless act," Borgzinner said. "I'm somebody trying to do right, living his life proper."

Police say Borgzinner suffered orbital bone fractures under both eyes and other injuries when four men beat him at about 5:45 p.m. Monday on the northbound bus near 11th and Howard streets.

Borgzinner, a graduate of the June Jordan School for Equity public high school, said he had been studying a monologue for the acting class he attends at night.

He started his acting career last year after he wrote and performed a monologue in a high school theater class. In it, his fictional character seeks comfort in prayer after standing by as his brother is shot and killed.

Teacher saw talent

A teacher who saw him perform the monologue knew of the planned movie "La Mission" and gave him the contact for the casting director. Two hundred actors auditioned for the part of Nacho; Borgzinner won the role. The film was shot on location while Borgzinner was a senior in high school.

Bratt stars as a felon struggling to overcome his history of violence and accept his son's homosexuality. The movie, directed by Bratt's brother Peter Bratt, was recently previewed at the Sundance Film Festival.

As Borgzinner studied his monologue in the back of the 9-San Bruno for acting class, a man started talking to him, asking if he was a 24th Street gang member.

Borgzinner told police he thought the problem might have been that he was wearing red tennis shoes, suggesting he was a Norteño.

Borgzinner said he wasn't a gang banger, and the man seemed satisfied. The two even talked about what high schools they attended.

Then the man asked to borrow Borgzinner's cell phone, saying his own was dead. Borgzinner overheard him ask, "OK, red light or green light?"

Gang slang

Green light is gang slang for approving an attack on a rival.

"Why did you lie to me?" the man then asked. "You are from 24th!"

Four men proceeded to beat him and steal his iPod Nano and wallet, he told police. Borgzinner was treated for his broken bones and other injuries at San Francisco General Hospital and released.

Police have made no arrests. A spokeswoman, Sgt. Lyn Tomioka, said the gang task force is waiting to see whether Muni can supply video from surveillance cameras on the bus.

Muni has not always been able to do so. When 11-year-old Hatim Mansori was stabbed by a stranger on the 49-Mission bus Sept. 1, it turned out neither camera onboard had taped the attack - although one of the devices captured audio of the boy's screams.

Muni later conceded that a "significant number" of cameras aboard the buses didn't work, but said that repairs were under way.

News expected today

Muni spokesman Judson True said officials would know by today whether cameras on the 9-San Bruno bus had been operating and whether the attack on Borgzinner had been taped.

The driver told police he had not seen the attack, but True added: "Our drivers are not supposed to intervene in any violent incident." Instead, they are supposed to call police.

Monday's incident was not the first time Borgzinner had been victimized. In January 2008, he said, before he was cast in "La Mission," he was attacked by gang members at 16th and Mission streets while he was on his way to a poetry slam. He said he channeled his anger from that attack when he auditioned for the part of Nacho.

"I kept that rage inside," Borgzinner said. "I incorporated it in the part."

He said he has since been asked to play gang members in other productions, but has refused. Nacho was not a gang member, but a street thug whose character learns lessons about the consequences of violence, Borgzinner said.

"I don't want to express Latino culture as having to be a gang member all the time," Borgzinner said. "I am an actor, trying to represent the positive look for Latinos."

E-mail Jaxon Van Derbeken at jvanderbeken@sfchronicle.com.