Most of the time, the best instruction in a studio session is one word long.
“Again.”
Sometimes, “You can do it better. Again.” is more effective, but mostly the one word response is the way to keep the flow going. A good percentage of the time, you can skip speaking entirely and simply start the track rolling again.
My impulse is always to give more advice, but truly, if singers are prepared for the session, you don’t need to give them a lot of instructions. Just keep them moving. If you do it right, singers will get into a flow state where they can be productive and excellent very quickly with little down time. If you do it wrong (talking a lot!), singers are likely to get in their heads and decrease in productivity and quality.
It’s true outside the studio, too. We don’t always need lots of instructions. Sometimes we do, but a lot of the time, we can just be told to try again, to do it better.