I think you know people better than you think you do. Some things are just logical. For example,
Betray a secret someone told you. Will they get angry? Oh, yeah. How will that anger manifest? Hard to say. They might betray something in return (something you REALLY didn't want known). They might slash your tires. They might egg your house. They might start a bad/false rumor about you. You know this is true but it's up for you to determine how your CHARACTER will respond.
Here's a helpful list someone once gave me that's really helped develop a character's backstory so you know how they'll respond to a given stimuli:
Answer these questions for each character you need to make rock on the page (don't worry if you're not writing a contemporary character. Just change the "apartment" to "cave" or "rocket" and the influences of society into those in your world):
1. Character Name.
2. Where did character grow up?
3. Choose three clubs/sports the character was involved in in high school (if they went to school.)
4. What is a quirk (such as spinning hair around a finger when nervous, etc.?)
5. When decorating an apartment, where would your character shop?
6. What role does money play in your character's life (is a 20" b/w television just as good as a 50" HD flat screen?)
7. Inspiration for Character (i.e., character from a movie, fairy tale, story, etc.)
8. Possible physical features like scars, tattoos, etc.
9. How do you see the character (i.e., sterotype, caricature)
10. Possible conflicts in personality (i.e., likes to watch sports, but hates to PLAY them.)
11. Possible need for change.
12. Values and beliefs (church-going, would the character steal if starving? )
13. How beliefs and values clash (would the character steal if sufficient reason? What is that reason?)
14. What do they need in a mate?
15. Who is the worst person for them to fall in love with?
16. What makes the character emotionally dangerous (seeing someone strike a child, etc.?)
17. What is it about the character that makes it impossible for him/her to simply "walk away" from the crisis of the plot?
18. What does the character most admire about their best friend?
19. What drives the character insane about their best friend?
20. How does the plot help the character learn a lesson or grow?
21. What is the error in thinking during the plot (i.e., thought they could trust someone untrustable, so didn't spot the danger, etc.?)
22. Why do they hold this belief?
23. As a result of this belief, what do they need to learn?
24. What is keeping them from learning it?
25. What are the ways the character tries to "cheat" to keep from having to grow?
26. What event in the external plot forces the character to either grow or change?
27. Pinpoint your character's greatest fear.
28. What is your character's greatest secret?
29. What is your character's best childhood memory?
30. What is your character's WORST childhood memory?
These are the sort of things that Shakespeare was brilliant at. He introduced just enough of the character's past to let the reader know they have one. The prime ingredient in any good book is the reader's belief that before the book opened, the character existed: they were born, they had a childhood, friends, enemies, family, sucky things happen, terrific things happen, etc. The crisis or point of the book is going to interrupt that life.
Good luck!
