I read a novel as a teenager in the early 2000s about an alien invasion. What I remember is the main characters were teens, the aliens were blobs that entered people's heads, and the teens described seeing them behind their parent's eyes. Those who were taken over didn't have emotions and would communicate with others without talking. I'm pretty sure it was part of a series. Anyone remember this novel?
I believe you are referring to the Visitors series by Lynn Harnett and Rodman Philbrick. This is a scifi horror series for youths about teens surviving an alien invasion. These aliens do what you describe - they take over human hosts by entering their heads, and infected humans are described as having something living behind their eyes. The summary for Book 1 at the GoodReads link also mentions the infected talking like robots.
It could also be the Animorphs series - there is a covert alien invasion by slug-like parasites called Yeerks. A group of teens inadvertently acquire the ability to shapeshift into whatever animal they touch, and they do battle with these aliens over the course of a whole novel series. The Yeerks communicate with each other via telepathy so there's no need for possessed people to talk to each other, but they are also very cold / emotionless.