Source Code : Good acting but a movie is only a movie

How could someone, being blown up numerous of times, ( I imagine he could feel the pain) and after realizing he is deceased act so cool and calm every time he goes back to the train sequence? I mean, come on this guy has just died, yet he acts so carefree and easygoing every time he talks to the people on the train, he doesn't shows any sign of fear or depression after experiencing death so many times painfully and realizing he is already dead man living on his mind. He is a soldier not an actor so it doesn't make sense at all. Even the most professional person cannot feel calm in a situation like this.

I just watched the movie and he freaks out quite a bit during it. He is always tense and on edge when on the train. He is focused because he has a job to do. After dying so many times, he just kinda gets used to it.

When he is not on the train, he is freaking out and screaming at the military looking for answers/solutions.

I am not sure how you missed all this.

He is not tense he jokes and mock the people on the train sarcastically.