I'm curious to see how well this is adapted into a film as I feel like a lot of the elements that are supposed to help build a sense of unease weren't very effective in the book and I'm curious to see if they prove more effective in a visual medium.

I can see why this story has been well received by a lot of readers, but, unfortunately, it wasn't a good fit for me.

The two main reasons for this were:

1. I quickly picked what was going on,

and,

2. I have a personal dislike of books that use mental health disorders as a "gotcha" twist, especially when the author isn't  writing from a place of experience with the disorder in question. 

Reid misrepresents dissociative identity disorder and, in doing so, reinforces the stigma and stereotypes surrounding it.

For instance, people with DID do not visually perceive their Alters and do not interact with them outside of their Headspace. Visually perceiving an Alter could occur as a result of a psychotic episode involving hallucination, which may be what is occurring in the story, but regardless of whether that is the case or not, I still don't approve of the author exploiting a legitimate mental health disorder for profit.

The only positive is that the character isn't portrayed as being a danger to other people, which is a common trope when it comes to DID.

Overall, I like what Reid intended to do with creating a sense of unease from subtle interactions, but I personally don't feel like they were very successful in that regard.