Constructing a sound screenplay takes a lot of concentration and there is some multitasking involved. Sometimes we can get so stuck in our plot and formatting mechanics that we put character development to the side; thus the characters acting only as bodies moving around.
Something to keep in mind when reading back your completed script is, "Have my characters changed the ways I wanted them to." Some characters are constants and do not change (these are good for opposition), but your players should experience adversities that enable them to change for better or worse.
Remember to have solidified starting and end points for your characters. They need to go through transformation that spits them out in the end.
Bottom line, keep this front of mind. It can slip away without you even realizing it until you go back and read a bland character that stays stagnant throughout the entire story.