Some of my favorite details in "thoroughbreds-2017"
In the hugging scene, you have the obvious detail of Lily inhaling Amanda's scent after Amanda confessing she hadn't showered for a few days. But I noticed that Amanda looks the same as Lily in the last frame we saw her when we see her for the first time in the next scene. Also her immediately showering after Lily mentioned it.
The first and last time Amanda shows any emotion (fake or otherwise) both happen when she looks at a picture of Lily
Lily literally drowning herself to see if Amanda would save her.
Amanda's poker icon being horse related.
Amanda and Lily both having the same picture, only Lily's version has Amanda cropped out. I think this represents that Lily is important to Amanda, but Amanda isn't important to Lily. Plus Amanda's version is worn and folded, and Lily's is pristine and perfect, representing their differing lifestyle. Also only Amanda is seen to ever look at them.
Amanda playing chess by herself while Lily sits on the side. To me this shows that Amanda is just another piece in Lily's game while she keeps herself safely away from it. Also, Amanda using a pawn, and then a knight to open the other side. As far as I can tell, that opening move is called "the king's knight." Which is kind of what Amanda is. Lily is the king, and Amanda is her knight.