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I wrote back before Christmas about the principle of singling out individuals in rehearsal - when and how it is useful, when and how it may be counter-productive. As I worked through the different scenarios and examples it got me reflecting on, I realised I was mapping the focus on individuals vs group onto the Yerkes-Dodson curve that charts arousal and performance level.
A focus on individuals makes everybody feel slightly less psychologically safe - not just the people singled out, but everyone else too as they become more aware that they too are individually visible. This results in an increased level of arousal.
If the choir is being a bit dozy or passive, then this is exactly what you need: a bit more alertness and focus in the brain, a bit more energy and readiness-for-action in the body. If the choir is anxious or floundering, though, this is the last thing you need, as adding adrenaline to an over-stressed performance just makes it worse.