The case is expected to put the "30 Rock" actor in an unsought role: a key witness against a woman who says she had a fling with him some years ago, which he denies. Baldwin is expected to testify next week against actress Genevieve Sabourin, whose lawyer says she was merely trying to get answers after the actor lost interest in her.
But prosecutors say Baldwin was alarmed by a campaign of calls, text messages, emails and unexpected appearances by a woman he'd met twice over 10 years - a creepy crusade that escalated as he got engaged to now-wife Hilaria Baldwin, authorities say.
"There was nothing legitimate about these communications," Manhattan Assistant District Attorney Zachary Stendig said in an opening statement. A judge is hearing the case without a jury.
Sabourin, 41, hails from the Montreal suburb of Candiac and has appeared in some Canadian films and TV series. She met the Oscar-nominated Baldwin on the set of the 2002 sci-fi comedy "The Adventures of Pluto Nash"; he had a cameo, and she was a publicist.
He told police that they had dinner together in 2010 and that their relationship was strictly professional.
But Sabourin harangued him with so many phone calls that he changed his number, and she sent emails that ranged from "I want to be your wife now" to a bid to "conceive a mini-Baldwin," Stendig said.
Although Baldwin asked her to leave him alone, she appeared in March 2012 in his driveway in Amagansett, N.Y., but left before police arrived, according to prosecutors.
A few days later, after news of Baldwin's betrothal broke, Sabourin turned up at a Manhattan movie screening he was hosting; security guards threw her out, police said. Then she showed up outside his Greenwich Village apartment building on April 8, 2012, with her dog and a car loaded with her possessions, and was arrested on misdemeanor stalking and harassment charges.
Sabourin and her attorney, Todd Spodek, said her contacts with Baldwin were far less one-sided than prosecutors portray.
"It was a communication, both ways," she said outside court.
She and Baldwin exchanged emails about matters as personal as her father's death, and in 2010 he took her for a romantic dinner and accompanied her to a hotel for a sexual tryst, Spodek said.
"You will hear about all of this attention given to Ms. Sabourin throughout this time period, and then, suddenly, the cold shoulder," Spodek told the judge. Her subsequent conduct was simply "looking for closure," not a crime, the attorney said.
If convicted, Sabourin could face up to 90 days in jail on the top count alone.
Baldwin's publicist declined to comment on the case.
Baldwin, 55, and his wife married in July 2012. Their daughter, Carmen, was born in August.
The eldest of acting's four Baldwin brothers, Alec Baldwin's long list of credits range from a Tony-nominated performance as Stanley Kowalski in "A Streetcar Named Desire" on Broadway to playing the action hero Jack Ryan in the 1990 film "The Hunt for Red October."
Baldwin recently launched a talk show, "Up Late with Alec Baldwin," on MSNBC.