When the Corleones are sitting around waiting for Sonny's contact to let them know where the meet with the Turk is going to take place, we see Tom begin to unravel a bit as the time of the pickup gets closer and closer.

It's almost as if Tom is starting to panic.

Is Tom sincerely worried about Michael's safety?

Is he more worried that the plan will fail and that he'll have to explain himself to the Don if anything happens to Michael?

Or did he have doubts about Michael's plan all along and began to show his doubts at this point because everything was about to go down?