Berg may have been calm in the video because his hypothetical Al Queda buddies told him it was just going to be a staged propaganda film to show that they had a prisoner. They may not have told him they were going to kill him. Maybe they said they would add in "special effects" work later to make it look like he was killed. Since in this scenario he was working with Al Queda, he would have had no reason to be scared until it occured to him they were really going to kill him.
Bingo. Just like the Japanese "hostages" who were in fact "peace activists" of the extreme left variety. Like ANSWER.