Watchman Eddie Cuizon told investigators he was checking his neighborhood in central Cebu city for armed men when he tried to confront two masked men on a motorcycle and opened fire as they sped away - unaware that they were actors.
Police investigator Roger Nedamo said Cuizon was charged with homicide and violation of a gun ban in the death of actor Kirk Abella, 32.
The gun ban was in place because of a local election. Normally, watchmen are not allowed to carry firearms in Philippines, where many street killings, including those targeting political activists and journalists, are perpetrated by motorcycle-riding gunmen.
Abella was playing a vigilante in the low-budget movie "Going Somewhere" by British theater and film director Alan Lyddiard.
Dozens of people watching the filming late Saturday thought the gunshot they heard was part of the movie, said Parian district community police chief Alexis Relado.
Lyddiard's Filipino assistant, Beverly Tanedo, said in a cell phone text message to The Associated Press that they were "baffled why it happened, having taken the necessary precautions and have asked the help of the police for security."
"Our immediate concern now is to help Kirk's family get through this," she said, adding that they were helping the family to hire lawyers.
She did not respond to queries on plans for the movie.
Assistant Prosecutor Noel Sellona, who will review the police complaint before deciding whether to file a case in court, said the family wanted Cuizon to be charged with murder instead of homicide, a lighter crime punishable by up to 20 years in prison.
But police opted for homicide charges after determining the shooting was not premeditated.
"It wasn't planned. It was plain stupidity," police investigator Nedamo said.
Victor Sablada, who played the motorcycle driver, said Cuizon introduced himself as a police officer. "I told him, 'Buddy, we are just shooting,"' he said.
Sablada said that as he revved up the motorcycle and started to move, Cuizon jumped on the seat behind Abella and shot him point-blank in the back. Cuizon fell off the motorcycle and Abella, mortally wounded, slumped to the ground, his body hitting a lamp post, he said.
Cuizon, 52, told police he had been woken up by a concerned citizen who alerted him that armed men were seen around the neighborhood. Volunteer watchmen help police keep the peace and patrol the streets. They are normally armed only with batons and are not supposed to carry firearms.
Abella was dead on arrival at a hospital. Realizing what had happened, Cuizon surrendered to police. He was remorseful and wanted to seek forgiveness from Abella's family, police chief Relado said.
Abella's sister Cleofe Escanillan said her brother was a bit player and worked occasionally as a waiter.