If the cohein gadol was a knowledgeable person, he would talk and if not other talmidei chachamim would talk before him. If he knew how to read tanach, he would read and if not others would read before him.

They read before him Iyov, Ezra and Direi Hayamim.

Zicharya ben Kivutal said, ‘Many times I read before the cohein gadol from Daniel.’

Yuma 1:7

If the cohein gadol started to fall asleep, the young cohanim snapped their middle finger and they told him stand on your feet and get cooled off.

The cohanim were busy with the cohein gadol until it came time for the slaughtering of the animals.