The actor Alec Baldwin was filming the movie “Rust” in New Mexico in 2021 when the gun that he was rehearsing with, which was not supposed to contain live ammunition, went off, firing a bullet that killed the film’s cinematographer, Halyna Hutchins. The film’s armorer, Hannah Gutierrez-Reed, who was responsible for weapons and ammunition on the set, was convicted of involuntary manslaughter and sentenced to 18 months in prison. Now Mr. Baldwin is going on trial for involuntary manslaughter; he has pleaded not guilty. Opening arguments begin on Wednesday. Here are some key players.

The ‘Rust’ Production

Alec Baldwin

Actor and producer

Mr. Baldwin, who was playing a grizzled outlaw in “Rust,” has vehemently denied responsibility in the fatal shooting on Oct. 21, 2021, saying that he was told that the old-fashioned revolver he was handed on the set that day was “cold,” meaning that it was not loaded with live ammunition, and adding that it was unthinkable that any live rounds would be on the set. Mr. Baldwin has also said he did not pull the trigger when the gun discharged, but had merely pulled the hammer back and let it go; prosecutors have said that forensic examinations have suggested that he must have pulled the trigger.

Hannah Gutierrez-Reed

Armorer

As the armorer, Ms. Gutierrez-Reed was responsible for weapons and ammunition on the “Rust” set; even though there was not supposed to be any live ammunition on the set, she loaded a live round into the revolver that day and failed to catch it when she checked the weapon. She stood trial this year, and a jury convicted her of involuntary manslaughter. Prosecutors argued that she had brought the live rounds onto the set, which she denied. She was sentenced to 18 months in prison — the same maximum sentence that Mr. Baldwin would face if he is convicted. She is appealing the conviction.

Dave Halls

First assistant director

As the first assistant director, Mr. Halls was supposed to try to keep the film on schedule and was also its safety coordinator, with the responsibility of checking firearms to ensure that they were safe to handle. Mr. Halls has admitted that he failed to thoroughly check the revolver on the day of the shooting. He agreed to a plea deal on a charge of negligent use of a deadly weapon and avoided prison time.

Sarah Zachry

Head of props

Ms. Zachry was responsible for props on the set, including guns and ammunition, and was Ms. Gutierrez-Reed’s supervisor. She has acknowledged that she was responsible for one of two accidental gun discharges on the “Rust” set five days before the fatal shooting. She signed a cooperation agreement with prosecutors that protects her from prosecution as long as she testifies truthfully.

The Court

Mary Marlowe Sommer

Judge overseeing the trial

Judge Marlowe Sommer of the First Judicial District of New Mexico has a deep familiarity with the facts of the “Rust” shooting after presiding over Ms. Gutierrez-Reed’s trial. She handed down the maximum prison sentence in that case, telling Ms. Gutierrez-Reed in court that she was not willing to give her a “pass” that she did “not deserve.”

The Prosecution

Kari T. Morrissey

Special prosecutor

A career defense lawyer, Ms. Morrissey was appointed to prosecute the “Rust” case after the previous special prosecutor stepped down and the Santa Fe County district attorney stepped back. At first, Ms. Morrissey dismissed the case against Mr. Baldwin, saying she needed time to investigate concerns raised by his lawyers that the gun might have been modified in such a way that made it more likely to accidentally discharge. After forensic gun experts analyzed the weapon, Ms. Morrissey said she became convinced that Mr. Baldwin must have pulled the trigger and decided to revive the criminal case against him.

Erlinda O. Johnson

Special prosecutor

Ms. Johnson is a new addition to Ms. Morrissey’s team. A former prosecutor, Ms. Johnson has a law practice in Albuquerque, at which she deals with personal injury, immigration and civil rights cases.

The Defense

Alex Spiro

Lawyer for Mr. Baldwin

A New York defense lawyer and former prosecutor known for his aggressive approach, Mr. Spiro has a high-profile client list that has included Elon Musk, Jay-Z and Mayor Eric Adams. He has been leading the arguments in many of the hearings that took place before the trial, frequently objecting to the prosecution’s conduct in the case.

Luke Nikas

Lawyer for Mr. Baldwin

Mr. Nikas, a lawyer who is best known for litigation in the art world, started representing Mr. Baldwin in the “Rust” case several months after the fatal shooting, when civil lawsuits were being filed. Mr. Nikas and Mr. Spiro had previously represented Mr. Baldwin in a civil dispute that arose after a scuffle over a parking spot.