There's always one player that wants to jump in to the center of attention. Sometimes there's more.
You know that player. You tell one player that he discovers a chest in the corner and this player screams he/she is going to open it, even though he/she isn't the one who found it. Or he/she is in the back stealing the gems out of the eyes of the golden statue but when another player locates a secret door on the other side of the room, he/she announces he/she is listening at the door.
He/she is an spotlight hog, and his/her behavior needs to be moderated.
Originally I used to go around the table and ask each individual what they were doing before I started feeding them the results of their actions. That way the spotlight hog had to decide what they were doing for themselves instead fo jumping all over another player's actions.
"Ooh, I open the chest!"
"It's not your turn and you aren't the one who found the chest. Besides, you are spending time on those eye-gems, remember?"
It worked fine until one of my players mentioned that the most covetous seats at the table were next to me since I always started at one end of the table and that often times I started on my left. So the next time I ran and it got to the heroes doing exploration, I dealt initiative cards.
At first it freaked them out as they thought combat was starting. My spotlight hog player quickly started shouting all the preparations she had made, trying to get it out before her card was dealt. I told her that didn't matter but she insisted she was ready for combat. Then we had a discussion about her meta-gaming (it fell on deaf ears).
But the initiative cards did help. It allowed each to player to have a moment to announce their actions and it got rid of any accidental favoritism on my part.
So if you find that your having a hard time managing the table, either because it is too large or because you have spotlight hogs, I recommend using initiative cards out of combat as well as in.