1. Darger wrote a giant book and painted many many pictures for it, called In the Realms of the Unreal. As the movie progresses through his life and what happened to him, we see his reaction in that part of the book. There are a couple of odd things in the images -- for example, the children are almost all girls, and they are frequently naked. The naked girls have penises. There's some conjecture whether he ever saw a naked girl/woman, but some think he was trying for ambigender. Also, the bad guys wear Confederate uniforms and "professor hats."
2. He didn't know how to draw when he started the book. He traced, cut, collaged, make masks, and stencils. This was very interesting because he was so determined to draw that he kept practicing.
3. The people who were closest to him as an adult frequently believed very specific things about him -- like where he sat in the Catholic church for three masses a day -- and they would all be different. This set of contrasts went with his memoirs which pictured him as outgoing and a good would-be adoptive father while he was very retiring and had been turned down for adoption.
His landlords found all his materials the day before he died and were shocked at what he'd been doing in his room all those years. They made them public and now his work is in galleries.
This is a really intriquing movie -- the more you learn about him, the more you wonder. When you think you've figured something out, something changes. If you're interested in folk art, personal history, and/or the comparison of real life with mind life, this is your movie.