I believe that the reason for Tom's calm and cool demeanor, which is almost a facade for the type of person that he really is, is that he was trained and schooled by Vito on how to keep his temper, to be reasonable, and when called for, cold and cunning. When you think about it out of all the children, Tom was probably with Vito more than any of them.
Not sure if anyone in here has ever put it like that but I agree 100%. He was Vito to a fault, he became TOO calm and cool, he didn't realize that sometimes you have to roll the dice and break the mold a bit. Luckily, Mike did. Thus they were able to eliminate McCluskey and Sollozo despite Tom's initial cautious reservations against the plan.
I think Tom's character has one of the greatest developments in the movie though as he learns as much from Mike as he did from Vito.
