The "30 Rock" star said at the Manhattan trial of the woman, Genevieve Sabourin, that he had a meal with her at a friend's request, gave her an email address and phone number and later communicated with her about acting classes.
She began to barrage him with as many as 30 voicemails per night and hundreds of emails that started out as lovelorn and pleading, Baldwin said. They escalated into ominous threats to insinuate herself into his now-wife's yoga class and to come to his homes, even as he implored her to stop and threatened to call authorities, according to Baldwin.
"It was nightmarish," he said.
Sabourin had some outbursts as he testified, including: "You're lying!"
Sabourin says she and Baldwin had a 2010 tryst and a two-way exchange of emails. Her lawyer, Todd Spodek, has said Sabourin wasn't stalking the actor, just seeking closure after he stopped talking to her.
The Oscar-nominated Baldwin and Sabourin, who has appeared in Canadian movies and TV shows, met when he had a cameo and she was a publicist for the 2002 sci-fi comedy "The Adventures of Pluto Nash."
Fast-forward to April 2012, when Sabourin was arrested outside Baldwin's Manhattan apartment building.
Baldwin got engaged to and married the former Hilaria Thomas in 2012, after they began dating the year before. They had a daughter, Carmen, this past August.
Hilaria Baldwin also is expected to testify.
Baldwin's career has included a Tony-nominated turn as Stanley Kowalski in "A Streetcar Named Desire" on Broadway, incarnating action hero Jack Ryan in the 1990 film "The Hunt for Red October" and an Academy Award nomination for his supporting role in the 2003 drama "The Cooler."