Sera Gamble and Caroline Kepnes had just wrapped a long day on the set of their new Lifetime romantic thriller, “You,” and all they wanted to do was celebrate.

It was August 2017, sometime between midnight and last call. Ms. Gamble, a writer and producer who developed “You” with Greg Berlanti, and Ms. Kepnes, who wrote the novel it’s based on, were in New York City, where the show is both set and shot. Production had just begun on the series, which will premiere Sept. 9, and the women had decamped to a bar for a hard-earned drink and soon became engrossed in a conversation about their characters. Then, as Ms. Gamble recalled, “This quite drunk guy sits down next to us and just starts talking.” His topic of choice: Donald Trump.

“He wasn’t even friendly!” Ms. Kepnes said, laughing, at a Midtown Manhattan hotel just after Labor Day. She and Ms. Gamble, who don’t get to see each other as often as they would like, had reunited that afternoon to discuss their collaboration, often finishing each other’s sentences in the process.

Ms. Gamble recounted how startled she, an accomplished kickboxer, had been by her and Ms. Kepnes’s timidity when faced with the stranger at the bar. “We’re really sweet to him, and we say, ‘You know, we don’t get to see each other every day, and we’re just having a little conversation about work right now,’” she said. “And he didn’t get the hint at all.”