Artemis shows his respect to the royal family by following the others and standing while they enter the vast hall. He watches the rest of the room out of the corner of his eye and sits when the others do.
Clearly there were many customs and protocols to be learned. For now, he would take cues from the others gathered in the room who undoubtedly were more accustomed to such things.
After the King made a toast, drained his goblet, and placed it upside down on the table, others in the room did the same. Assuming it was a social custom he had never heard of, Artemis raised his own goblet, saluted the royal family, made a brief toast to their health and prosperity, and drained it before placing it upside down on the table. It seemed to be a most unusual custom but since he was here, he would be expected to follow whatever strange rituals and customs they had.