I feel Ethan Embry is not convincing

Ethan Embry as Kevin has some moments that stayed in my mind. For me, it does not feel very real in many scenes. The role needs more detail, but the performance stays on the surface. Many moments pass very fast, and I do not remember much after them. Some emotions are too simple, and I cannot believe the character enough. This is only my personal view, but the performance changed how I enjoyed the film.