Eliot: We have depth. And... character.
Margo: [looks horrified]
One of the basic traits of a character is their depth, or how complex they are. Some characters are two-dimensional Flat Characters, with one or a few defining traits that completely define who they are. Rounded Characters go beyond being a stock character with some Backstory and Hidden Depths. They generally have a complex motivation driving them, and may act counter to what their 'type' would suggest. These are the kind of character that have to struggle with choice.
A Flat Character may evolve into a three-dimensional character if they are dynamic, and change according to what they experience. On the other hand, sometimes they don't need to change and remain Static Characters.