I don't get into my characters heads, they get into mine. 
It's really weird, but for my more developed OC's, it feels like they completely take over when I'm writing a scene. I know how they think, so I know how they'll react. 

Dialog comes naturally to me, writing expressions/body language is more difficult because I'm still a little rusty. Because of this, I usually plan dialog first, and go from there. :D