Just me thinking and regurgitating what Iv read and somewhat noticed on my own..
Characters are defined by their actions. The sum of the decisions made, regardless of the "characterization" define the DEPTH of the character.
From Robert McKee
"Character Versus Characterization
CHARACTERIZATION is the sum of all observable qualities of a human being, everything knowable through careful scrutiny: age and IQ; sex and sexuality; style of speech and gesture; choices of home, car, and dress; education and occupation; personality and nervosity; values and attitudes - all aspects of humanity we could know by taking notes on someone day in and day out. The totality of these traits makes each person unique because each of us is a one-of-a-kind combination of genetic givens and accumulated experience. This singular assemblage of traits is CHARACTERIZATION... but it is not CHARACTER."
Working from the bottom up..
Some characters performs actions that fit their characterization.
Example: A guy dressed like a waiter puts plates on a table.
Some characters act CONTRARY to characterization.
Example: A hit man interrupts a "job" to help a little girl with her homework.
The more MEANINGFUL the contrary action (contrary to Characterization) the more DEEP the character and the more LEADING the role.
Bit parts have little no contrary actions
Supporting roles might have some contrary actions.
Lead Role has the most.
If your supporting character is revealing deep inner life by contrary actions, then maybe its not a supporting role. Make that character "less" complex, or make you lead character more so.