I’m sure it’s possible to be over-rehearsed, but it isn’t something I experience regularly.
In a Real Book group, it’s far from uncommon to be ten seconds before downbeat and get a quick rundown: “Last four, take the coda. Two, three, four…” Some decisions are made in-flight, such as who is going to play the head and whether the bass will take a solo. That’s good, because jazz is in the moment.
It seems chaotically loose, but it works because of two things: defaults and eye contact. In the quintet, we’ve been playing together a long time, and we know the default roadmap–who takes solos and in what order. If nobody says anything, we’re fine and nothing needs to be said. However, during the intro I can give the sax player a look that says, “You go ahead and take it.” I might actually mean, “This sounds better on sax,” “I hate this song,” “I just swallowed a fly,” or “I don’t have my sheet music”, but those all boil down to the same result. (It gets ugly when we both give each other that look.)
We can do more than that. We have a rudimentary language that allows us to use hand signals or even lip-reading, and for that matter we can actually talk (wind players when not playing, rhythm players most any time), though we don’t much. So in a group that’s been playing together for a long time, I can raise an eyebrow and find out if we’re splitting the head AA/BA. A nod from the piano player can mean “another chorus” or several other things depending on context. It’s hip, dig?
Even when we are following the standard form, as trumpet player I spend a lot of time not actively playing, so I go ahead and double-check. We generally end solos with a physical turn to the next person, and something I learned from theatre is to step out of the way and focus my gaze on the person playing to direct the audience’s attention where it should be. I look at the next person during each solo, wait for them to look up, and get visual confirmation. When I get a signal I make eye contact with the others to be sure we all go there together. There’s a whole different set of signals for communicating with the sound man (“The singer two people to my left needs to be higher in the center monitor”).
The other thing I do with my eyes is watch other people play. This has a number of uses, including but not limited to learning new licks, enjoying the show, checking where we are in the changes, and realizing that my copy of the music is in the wrong key. It’s good to memorize music and/or play by ear. I mean, with all this going on, when do I have time to look at my stand?