A good way to do this is to position the character around the theme.
By giving the character a certain belief or point of view relative to the theme, you determine more specifically Who They Are.
This helps decide so many things: their backstory, their description, how they'll change, how they interact with other characters, their role...and so on.
It gives you a solid foundation for building a character, a solid role for the character to play and a solid reason for the character being in the story.
The THEME ADVANCED WORKSHEET is available at http://www.kalbashir.com/THEME-Advanced-Worksheet.html