What do you characters want?
What are they afraid of?
What’s at stake for them?
What are the consequences of their actions?
How do they speak and why?
Do a character sketch inspired by the physicality but not the interior life of an actual person. Note the unique details of your ‘model’ – mannerisms, posture, how they move their body, eye movement, the shape of their hands, speech, tone of voice, word choice…
Now, do an inventory of your character, filling in the interior life by asking yourself the questions above and then some.