A static character is somebody who doesn’t go through any major change during the course of the story. A great example of a static character in this novel is Barb, the respiratory therapist at Saint Grace’s Hospital. From start to finish, she cares for her patients enough to be a stickler for the rules.
Flat characters are defined as those who are not highly developed in the story and do not have much emotional depth. Stella’s friends, Mya and Camila, fit the bill here. Will’s friends, Jason and Hope, also do.
I would use Will as a great example of a round character. A round character is often one of the main characters in a story, which is applicable because Will is one of our protagonists in Five Feet Apart. By definition, round characters encounter conflict and are changed by this conflict. In Will’s case, this “conflict” is Stella, who persuades him to follow his medical regimen and to care about his own well-being.
Foil characters are added to a story to create a contrast with one of the other characters. The aforementioned friends of Stella and Will—Mya, Camila, Jason, and Hope—fit this category, because they do not have cystic fibrosis and are therefore in contrast with Stella and Will.
A stock character is somebody who reflects a stereotype. This character is generic, highly simplified, and not developed by the author at all. I would argue that there is no such character in Five Feet Apart.