I get a fair amount of that kind of work too, on bass. Mostly jazz, where the repertoire of "standards" is finite. I also go to fiddle jams, where the fiddlers themselves often have no idea what the harmony should be, or how to express that information.
Of course a trick that a layperson might not notice, is that the notes are nowhere close to random.
But I love the challenge of that kind of work if it's with good players.
Yep! yeah man playing jazz is I think the ultimate challenge for a musician. I made the mistake of trying to lean on fake books for a long time, I think because I didn't believe that it was realistic to memorize enough of the tunes for it to make a difference. Eventually I wound up with a teacher who insisted I do exactly that. I think it should have been obvious to me, but as I'm sure you know, once you have memorized maybe a hundred tunes or so, you develop this mental model that the bulk of all standards fall into on the basis of form, changes, feel etc. Then, even when its an unusual tune, maybe you have to struggle through the first A and B, some kind soul might take pity on you and shout out the changes, and that little bit of extra information is enough combined with all that other stuff you know to get you through the tune.
I guess all that memorization is like the conditioning and training that an athlete might do to prepare for an event, and then at the event you're mostly responding conditions.
As an odd coincidence, I ditched the fake books, almost 2 decades ago. It was kind of by accident. I had been hoarding more and more books, and got to the point of hauling a huge pile of books to every gig. One time, I got a last minute call, and couldn't remember where I had stashed the books, so I rushed out of the house without them.
And it went OK! In addition to the mental model that you describe, I've also learned that most tunes are more forgiving of changes than one might guess by all of the chord symbols notated so precisely.
I've revived my fake books for another use: To practice treble clef sight reading, which I neglected when I was younger. Also, I think learning melodies helps a lot in terms of mentally organizing and navigating the standards.
There are players who are a lot better than me, and are still attached to the books.
I get a fair amount of that kind of work too, on bass. Mostly jazz, where the repertoire of "standards" is finite. I also go to fiddle jams, where the fiddlers themselves often have no idea what the harmony should be, or how to express that information.
Of course a trick that a layperson might not notice, is that the notes are nowhere close to random.
But I love the challenge of that kind of work if it's with good players.