And it was when all eyes were fixed on the stage where Mr Devant was pouring out glasses of all sorts of different things to drink, out of one kettle with one spout, and the audience were delightedly tasting them, that Robert felt someone in that vacant seat.

He did not feel someone sit down in it.

It was just that one moment there was no one sitting there, and the next moment, suddenly, there was someone.
Robert turned.

The someone who had suddenly filled that empty place was Rekh-mara, the Priest of Amen! Though the eyes of the audience were fixed on Mr David Devant, Mr David Devant's eyes were fixed on the audience.

And it happened that his eyes were more particularly fixed on that empty chair.