I find the hot CIA chick totally unbelievable in her role. She is supposed to be some badass, but has a weak personality, lives with her mommy and constantly screws stuff up. Her romance with Billy seemed forced. She looks great in lingerie, though.
Also, the marine kid that goes psycho is an odd plot line and just seemed like time filler. It does bring some of the characters together, moves things along, but I think it could have been done better. I also just don't like the actor. I think the fat army dude who was lying and got killed was a well done character and I was conflicted when he died because I hated his guts, but it was a pretty gruesome death.
I do really like the David Lieberman character and think his wife, Sarah's, character is fantastic in her role. That is an interesting dynamic with him watching his family and them thinking he is dead. Really like the tension every time Frank goes over there.
The ex-marine guys are all OK, but not great. The black dude (Curtis) with the missing leg is the best of them. Billy is over the top and Gunner seemed like a fat but like-able hick. I pegged Billy as a bad guy after about the 2nd episode and that was one of the least surprising plot twists ever.