Mr Brancato says he was a bystander in the murder.
Sopranos actor Lillo Brancato Jr has gone on trial in New York, charged with the second degree murder of an off-duty police officer in 2005.
Prosecutors say the star, who played Matt Bevilaqua, should be convicted for his part in a burglary that led to the death of Daniel Enchautegui.
Jurors must decide whether the star was the partner-in-crime of a man convicted of the killing, or merely a bystander.
If convicted, Mr Brancato, 32, faces a possible life term.
The prosecution says the actor and 51-year-old Steven Armento, who was convicted of first-degree murder in October, tried to break into an apartment to steal prescription drugs after visiting a strip club together.
Mr Enchautegui, who lived next door, came out to investigate and Mr Armento shot the 28-year-old officer in the heart. Mr Enchautegui fired back, wounding both men.
"In their effort to get drugs to satisfy their own desires, they took the life of a New York City police officer," assistant district attorney Theresa Gottlieb said.
Mr Brancato's lawyer, Joseph Tacopina, told the jury his client was an addict who "ruined his life" with drugs and was "clearly strung out" at the time of the slaying.
But he argued that the true culprit was Mr Armento, who has been sentenced to life in prison without parole.
"Lillo was there to satisfy his addiction, but he wasn't there to do violence," Tacopina said.
"Lillo didn't try to hurt a single person that night."
By law, prosecutors were allowed to charge the actor with second-degree murder because they say the killing occurred while both he and Mr Armento were committing the crime of burglary.
Tacopina denied that Mr Brancato ever entered the apartment and said the actor did not know Mr Armento was carrying a loaded gun.
But prosecutors say Mr Brancato's drug dealer will testify that Mr Armento had pulled the weapon on the dealer earlier in the night in front of the actor.
Mr Brancato made his screen debut opposite Robert De Niro in 1993's A Bronx Tale and appeared in a number of other films before his role in The Sopranos.