If you have 15 people in your choir, chances are that you should set out 30 chairs.
Those empty chairs are for the "uninvited guests" who come along each week with your singers. They are the little ghostly voices chattering away at them - while you're talking, while they're singing. All the while they are scolding, correcting, intimidating, offering up memories of past mistakes - sometimes bringing even the most talented individuals to the point of melodic paralysis. And seriously limiting the effectiveness of your direction and your singers' abilities and happiness.
What's a director to do? You can't undo the damage of a failed jury exam, an ill-tempered high school choral director, a missed note during last Christmas Eve's solo, a mother who always pointed out how your sister was the one who could really sing, etc. No, you can't shoo other people's ghosts away. They can be quite powerful and there are very few musicians who don't carry at least or two around.
Sometimes we get so used to our ghosts' constant grumbling and kvetching that we just assume they are regular background noise. And the noise can get so loud that we don't hear what's really happening in terms of direction or our own vocal production.
What you can do is remind your singers of their existence and corrosive influence on their happiness and their singing.
Devise a gesture that will let each choir member chase them away, scoot them out of the choir room or toss them out of the loft. Tell those ghosts to be gone! Be imaginative, be a little physical. If nothing else, laugh them away for the moment. If you do this on a monthly basis, it will clear the air of those infernal spirits.
Then enjoy a rehearsal with the "real" folks in the choir! (And don't forget to chase your own ghosts away as well.)