Yep, like Mary said, seat fillers are common. That's actually what I did at the Sports Illustrated event, so I can't necessarily give the perspective of a celebrity. Here are some details that might help, though:
- All of the celebs were seated in the front section of the audience, while the people in the back were seat fillers. Celebs get up and leave the event all the time between breaks (when the cameras are not on them), so that might factor into your scene. If they aren't up for an award, they might go mingle in the lobby while the show goes on because they care more about networking/hanging out than the ceremony itself. If they are up for an award, they will probably wait until the award is announced.
- Seat assignments for celebs were specific, depending on a number of factors. For example, if they're expected to receive an award, they're usually seated close to the stage to limit transit time. Like Mary said, it's more common to be seated with your cast members if the event has round tables because it's easier to accommodate groups with them.
- Where a celeb goes after accepting an award depends on the show. For the one I attended, they had a picture taken of them while on stage, then walked back to their seats. For bigger events, there is normally an area backstage where winners go to get their pictures taken.
- Celebs are escorted from the red carpet to their seat by staff hired specifically for the event.
