SPOILERS!

Bertolt has nightmares every night about when they knocked down Wall Maria and the wall to the Shiganshina District. 

Reiner stays up for hours comforting him. He can't handle the grief of when he killed all those people.

Reiner tries not to remember it but it just keeps coming back into his head. He tries to cover it up with his happy attitude. This caused him to gain somewhat of a different personality(the one that talks about being a warrior).

Annie always has a bored expression on her face but really on the inside she's crying like a helpless child. She didn't want to kill all those people, especially Marco. The only reason she did it was because she didn't want to let anyone down and she had been training for that moment her whole life.