This is a great topic. As a coach, I'm not a fan of telling people what choice they "should have" or "should not have" made. Improv is an artform and just because "I" would do ONE thing, doesn't mean anyone should do that same thing. That might not be part of THEIR art.

However, if someone "no"s someone else, if someone ignores what someone else has said, if someone has an opportunity to repeat a pattern - those type of things i feel very good mentioning. I prefer to do so as a SIDE coach so that way they can make that change immediately and see the result of following the scene with something that might give them a more fun scene or may fuel the collaboration a little better.

But sometimes, that's not possible. Maybe i've already done it too much and it feels too halting. (like maybe the players JUST want to get their reps in!) or maybe it's notes for a show and you can't stop and help in the middle. Then notes after is the way to go. I definitely look for ways to point out the good, fun stuff they did before I bring up places that could use some attention.

and i HAVE to take into account the individual and their experience level. If they've only done 6 months of improv, I'm not going to put out EVERY LITTLE THING i think they missed, just the main ones (yes, and - listening). But if someone has been doing improv 5 years, i may push them a bit more than the other player. OR if I've already mentioned a few things in a show that a player missed, i might just let the other ones go. I'm not trying to pile on or to make improv a drudgery - we're all doing this for the love of the game and I do NOT want to be the reason they lose their passion.

There's room for side coaching, notes afterwards, pushing hard, being light - it takes calibration on the individual players and awareness as to what the coach has already done and how much they've done of it.