Jacob’s review published on Letterboxd:
It's often fascinating to see the results of an actor writing their own project. Typically these are more personal stories (at least on an emotional level, if not narratively), and their performance in the work tends to be remarkably grounded. Both of these qualities are present with Alison Brie in Horse Girl, an appropriately odd film about a socially awkward woman who begins to experience schizophrenic delusions. Her writing and performance of our protagonist are accomplished, giving her a completely realized identity outside of her illness, as well as a genuine commitment to the belief of the delusions. When Brie sits in a doctor's office and speaks of cloning and alien abduction, there's not an inch of embarrassment or light-heartedness; she so convincingly believes her own truth.
What appeals to me about the practice of an actor writing their own project is that it creates this deeper connection to the material while still leaving space for another artist to act as a layer of diffusion and perhaps highlight different aspects of the work than originally intended -- the director. That space for vision is a significant one, and that is why it's so significantly disappointing that Jeff Baena's direction is visionless. His visual language is two-dimensional, adding nothing to the text beyond a few nice shots on a zoom lens. There was a massive need for the delusion/fantasy sequences to be compelling visually, and they are anything but. They're the weakest moments of the film, and the final act as a whole suffers because of it. I won't say Brie's script was a masterful one throughout, as the lack of a satisfying ending likely existed on the page as well, but the way it's built in the film did it no favors.
Anyway, this is worth checking out, but if you're looking for a mentally-ill Alison Brie character with depth and humor, check her out as Diane on Bojack Horseman.