First, find out what your hero wants, then just follow him!
Do you know what your hero wants? Does she want power, love, money, revenge? Does he want freedom, success, joy, approval?
If you don’t know that, if you can’t articulate that, then you can’t understand their motivations. And you certainly can’t imagine how far they’d go to get what they’re seeking.
Make a list, one to five, of the things your character wants, with one being something they’ll do anything to get. (You might be surprised.) Now ask yourself, for each goal you list—how far will my character go to get that? And why?