http://variety.com/2017/tv/news/crazy-ex-girlfriend-rebecca-bpd-nathaniel-season-3-1202608629/

“We wrote her kind of by feel for a while,” explains Aline Brosh McKenna. “We kind of wrote her behavior in a way that felt right to us for about a season and a half, and then Rachel (Bloom) and I started to realize this was an area we were going into. It’s funny, we were trying to help Rebecca, so we were trying to figure out what was best for her. And when we looked into it, so much of this seemed to resonate.”

Crazy Ex-Girlfriend had doctors watch show episodes to diagnosis Rachel Bloom’s Rebecca character (https://www.vanityfair.com/hollywood/2017/11/crazy-ex-girlfriend-borderline-personality-disorder-rachel-bloom-aline-brosh-mckenna-intervie)
“We just said, ‘Watch these episodes of the show,’” says Bloom, noting that none of the doctors were told about Rebecca’s diagnosis.

http://www.elle.com/culture/movies-tv/a13938654/crazy-ex-girlfriend-borderline-personality-disorder-diagnosis/

“In a TV landscape littered with Manic Pixie Disordered Girls, there’s something admirable in the simple fact that Rebecca Bunch has a real illness—with a name and a treatment regimen,” says Sady Doyle. “But then, there’s the disorder itself. It’s progressive to humanize a mentally ill character. It’s a damn revolution to do so with Rebecca’s illness: Borderline Personality Disorder.” ALSO: Crazy Ex-Girlfriend sure loves to play fast and loose with the law (https://decider.com/2017/11/30/crazy-ex-girlfriend-is-a-legal-minefield).