Observe me, your overly dramatic twin sister. 

I can't say I'm exactly sure how to give advice on this subject. >.O I'd say the key is to empathize. Sit back for a moment, stick yourself in their shoes, while blocking out the rest of the world. How would you act? Then, how would /they/ act? 
Every character reacts to different situations differently. They'll feel different emotions based on circumstances, and it's sometimes hard to pick out exactly /what/ their emotions/reactions will be. 
Which is why it's essential to know a character inside and out, from their personality, past, where they grow up, who's around them, etc. All these things will have monumental impact on your character. I highly recommended character fractalling.  
Once you've got that down, just empathize.
(Though, I will say, from what I've read of your stuff, I haven't noticed anything particularly awry with your characters...seemed fine to me.)
Hope that helps. 
