At the end of the book, when R and Julie kiss, Julie's eye color changes.
How and why did Julie's eyes change from blue, to gray, to gold? Does the same thing happen to R? We can probably also infer that she can no longer cry, so what is she?
The kiss is hinted to infect Julie with the zombie curse, but then (at least in the book) they both magically reject the curse with The Power of Love, and the eyes of both of them turn gold. Hooray.
The film simplifies this by having R's eyes change color, from gray to blue.
After Perry is killed by R, R rescues Julie from the other zombies and takes her somewhere safe. Her reasoning for following a zombie isn't clear, and doesn't make a lot of sense considering her boyfriend was just killed by one. She attempts to flee from R the next morning, despite having numerous opportunities to escape at any point.
Julie does seem to be in a state of shock, as this is right on the heels of most of her party being killed in a zombie attack. She was probably too panicked to think of anything else to do at the moment.
Alternatively, she could have been confused as to why R was helping her but aware that if she started running away she'd "out" herself as being a living human, and possibly be captured and eaten by the zombies
In the world of the story, the more decomposed and frail you get ('Bonies'), the faster and stronger you become. Bonies seem far more efficient at hunting and killing, despite being reduced to nothing but bone and a thin layer of skin.
In the book at least the zombies are implied to be at least partly supernatural in origin. It's mentioned several times how the Bonies seem to vibrate with some weird energy. The fleshy zombies likely are likely some sort of intermediate stage where humanity and whatever animates them are at war until one side wins out completely.
R's friend, Marcus, as well as the group of zombies he's with, start to become human by seeing an image of a couple holding hands. However, R had to be completely smitten in order for his slow transformation to begin.
M is inspired by R's example, and the other zombies are inspired by M's example. As I recall, in the film at least R makes a brief mention of The Power of Love spreading quickly amongst the zombies. R's process was slower because he was doing it all by himself.
How does R's record player work? Does it have very long-lasting batteries?
Also, how long has the zombie apocalypse been going on? Julie, Perry, and Nora seem to remember the world being normal when they were kids, but the human survivor camp also seems fairly well established. Also, if it was a long time you would think all of the zombies would have turned into bonies by now, but that hasn't happened
It seems ambiguous, at least in the film, whether M actually remembers that his name is Marcus or if he just makes a snap decision to keep the conversation going.