@freemo I suppose part of a response should incorporate how 'professional' the respondents should be. If you're looking for people who derive *all* their income from music performance / composition, then I'm not qualified. But I'll give you my take, having played with others for a good part of my adult life, usually for pay.

Timing is pretty much everything. It's foundational to producing something listenable, vs cacaphony. It's the first thing we're taught, for those who have taken lessons. After awhile, it becomes innate. We count not only the beats, but the rests between them. We may use slightly different techniques, and that's fine, as long as the intervals are adhered to.

This also varies a bit by type of music. For example, symphonic music is much more rigid than more modern forms. You have people adhering to an as-written standard for every note and rest on the page, led by a conductor (which is another whole conversation as to his / her role in an orchestra). The players do know the piece from memory, having practiced the 'repitoire' for years before they got to that major orchestra. And, yet, they still follow the conductor for basic timing. S/he may make an error, but they'll all make the error together. I'll say that in most cases, the average listener won't detect errors (and part of being a good musician is knowing how to cover them up!) But they occur in every performance (especially live.)

Continuing in a more modern vein, you'll sometimes hear about the interaction between bass and drums, for example. Those two set the foundation, the table -- for everyone else in the group, so if they're not in sync with one another, you have what we colloquially refer to as 'a bucket of shit'.

Now, within that construct, bass and drums don't have to play notes or rests in the same places, and the most interesting pieces, imo, occur when they don't (think jazz, as an example). But they'll always 'come back' and sync up with one another. Actually, the argument I'm making is that they were never truly *out* of sync; it just sounded like they were.

Finally (for this post), we'll talk about bass / drums a little more, where they can play 'on the beat', 'in front of the beat' or 'behind the beat'. This is more the job of the bassist, and all have their place in music. When the two of them are 'locked in' together, the best way I can describe it is magic. A good rhythm section can carry mediocre soloists; a bad one can't make a band sound great, no matter how talented the others are.

Does that start to get to the point of your question?