SPOILERS BELOW
This shot within the first two minutes of the movie is of a window, where it's not clear if what we see is through the window of what's on the other side, or a reflection; which would look very similar, but would be inherently flipped.
This represents the fact that the reason Rachel is struggling to play Nora's character in that particular scene where she is speaking to her "mother" is because it's actually flipped, and it is actually Gustav speaking to Nora from her own character, which we as an audience also don't figure out until later. Later in the movie we can also tell that the shot of the window was actually not a reflection, showing that only people who grew up in the house (and know that it couldnt be a reflection) truly understand.
At the end of the movie, when we finally see Nora act out the script, the house we see is on a filmset, not in the actual house as Gustav originally intended (which had been sold), showing that this almost-curse like effect of the crumbling foundations of the house are now gone, alongside the estrangement between herself and her father. This is also explicity shown in the first scene of the movie around a minute after the window shot, with Nora staring at the crack through the wall (which halves the screen, resembling their relationship) as she covers her ears to drown out her parents arguing.
by Lzlxlclvlblnlmao