Nearly 18 months after a video of Daniel Penny fatally choking another man in a subway car in Manhattan spread online, a jury on Friday will hear the first official arguments about whether or not his actions were criminal.

Mr. Penny, a former Marine, is charged with manslaughter and criminally negligent homicide in the death of Jordan Neely, a homeless man who had a history of mental illness. Mr. Penny said he had stepped in to restrain Mr. Neely, who had been threatening passengers after boarding the train.

Opening statements will begin on Friday in the case, which centers on what happened as that F train traveled between stations and in the minutes after it stopped at Broadway-Lafayette Street on May 1, 2023. Prosecutors said Mr. Penny put Mr. Neely in a chokehold for nearly six minutes.

In interviews with detectives that evening, Mr. Penny said he had intervened because he felt that Mr. Neely “was absolutely killing someone” that day.

Prosecutors have argued that Mr. Penny’s actions became criminal when he refused to let go of Mr. Neely long after he had gone limp, after the train doors had opened and people had exited the subway car.

The opening statements will be the first chance for the lawyers on both sides to start laying out their cases to the 12 jurors and four alternates who will spend the next several weeks listening to witnesses and experts. The trial is expected to last through Thanksgiving.