The reason 'imposter author Julie' theory doesn't work for me...

I lean more toward the idea that Julie was imagined in Sarah's mind in the creative process of her writing her book, and that Sarah was in fact alone at the house the whole time - the entire film is very dreamlike and this theory makes more sense to me. Anyhow, others believe that the Julie who Sarah stays with at the house was some random woman posing as Sarah's publisher's daughter, and gained Sarah's trust and friendship in order to get her book published (albeit under Sarah's name) - in other words, there was no imaginary events and everything was straightforward and real. This theory of her being an imposter doesn't add up for me for a number of reasons, but mostly because of the end of the film. When Sarah walks past "Julia" at the end, she does not react in a confused or upset way. If she in fact saw the real Julie and had just at that moment realized that the Julie she thought she knew was in fact some random writer who had taken advantage of her, then you'd think she'd react differently than she does. She simply watches her from behind the door window, and slightly smiles. The final shot with the intercutting of "Julia" and "Julie" seems to signify to me a parallel between the two women, and how "Julia" was the inspiration for "Julie", the character in Sarah's novel, and how she transformed the real "Julia" (whom she vaguely knew as her publisher's daughter) into a seductive and scandalous character for her story.

"They say that time changes things, but you actually have to change them yourself."
- Andy Warhol