The involuntary manslaughter trial of Alec Baldwin began on Wednesday with prosecutors painting him as a headstrong actor who repeatedly shirked his duty to maintain gun safety on the set of the movie “Rust,” while the defense argued that the death of the movie’s cinematographer was the result of other crew members’ failures.

While Mr. Baldwin’s lawyers framed the 2021 shooting of the cinematographer, Halyna Hutchins, as a tragic accident, the prosecutors worked to convince jurors that it was a direct result of Mr. Baldwin’s recklessness while handling dangerous weapons on a film set.

Sitting at the defense table in between two of his lawyers, Mr. Baldwin — wearing a suit, patterned tie and thick-framed glasses — listened intently, sometimes taking notes on a legal pad, and looked somber as bodycam footage of the aftermath of the shooting was shown. Supporters sitting behind him included his wife, Hilaria Baldwin, and his brother Stephen Baldwin.

Here are four takeaways from the opening arguments in the first day of Mr. Baldwin’s manslaughter trial in Santa Fe, N.M.

The prosecution will try to show jurors that Mr. Baldwin repeatedly broke gun safety standards specific to the film industry.

While there is no video of the precise moment of the fatal shooting, there is a trove of footage from earlier in the production.

That footage, said Erlinda O. Johnson, one of the special prosecutors arguing the case, would show that Mr. Baldwin made a practice of putting his finger on or around the trigger when he should not have, in violation of Hollywood gun safety guidelines.