NEW YORK — A Canadian bit-part actress barraged Alec Baldwin with lovesick, demanding messages, packed her belongings into a car and showed up uninvited at his homes in Manhattan and the Hamptons, prosecutors said as her stalking trial opened Thursday.

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, is from the Montreal suburb of Candiac and has appeared in some Canadian films and TV series. She met the Oscar-nominated actoe or the set of the 2002 sci-fi comedy “The Adventures of Pluto Nash”; she was a publicist.

He told police that they had dinner together in 2010 and that their relationship was strictly professional.