Ari had a brilliant idea for a way to run chorus rehearsals. Soloist selection will be at random for each piece, but you won’t know if you have a solo until it’s time to sing it.
The corresponding conducting technique is to give the upbeat and then on the downbeat suddenly point at the soloist-to-be. This technique is adapted from a certain conductors-who-will-remain-unnamed’s habit of switching soloists at the last minute. The advantage is that everyone will be looking up out of their music, so it’s not all a wash.
An Ergodic Walk 
Well, clearly someone‘s already beat Ari to that technique.
You left out the part about it possibly being a member of the audience.
They actually do something very like this for the Christmas service of lessons and carols at King’s College: The boy soloist for the opening hymn (Once in Royal David’s City) is not made known until several seconds before he is to begin singing. The red light on the TV camera lights, Cleobury points his finger, and…
And they do this so the boys avoid getting nervous about the solo. Among other reasons.