Well, actually the guard only stands still for a short time. There is a routine where they march across the tomb, and then march back. If you read on it will give you the precise routine, which requires step counting and timing, as everything is done with significance.
It is drilled, drilled and drilled some more—and these young guards, by the time they are guarding the tomb. know the routine by heart.
I only saw it broken once. A crowd was not being quiet, and the guard stopped his march, faced the crowd, and said in a VERY loud voice “You WILL remain silent!”. He gave everyone in the crowd a stern look, and then resumed his paces.
Anyway, whether the guard is standing or marching, he'll be out there for two hours in the elements.