June 23, 2018, was a momentous day in the online life of Laura Pittenger, a New York playwright with roughly 1,200 Twitter followers. That was when the actor John Cusack, who has more than 1.6 million followers, retweeted her and followed her back.

Ms. Pittenger was happy to have a celebrity follower. Then came the anxiety.

“Honestly, I had a moment of paralysis,” Ms. Pittenger said. “I thought, ‘Why would he follow me?’ I was basically a nervous wreck that he was following me at all.”

Mr. Cusack, whose three-decade career has included starring roles in “Say Anything,” “High Fidelity” and “Hot Tub Time Machine,” followed Ms. Pittenger after she said, in a tweet, that the two of them shared a fondness for the antiwar activist Daniel Berrigan. Mr. Cusack had used a Berrigan quotation in his Twitter bio, Ms. Pittenger noted, and she had quoted the same line on her Facebook page.