Anyone who has seen “Bottoms” or “Shiva Baby” knows Rachel Sennott is irreverent, witty, and laugh-out-loud funny. And thanks to HBO, she’ll soon be front and center on her own series.

But she plays a supporting role in the film; the lead belongs to Rachel Sennott, making her feature film debut. (Sennott had another movie at festivals before Shiva, but it didn’t make it to ...