On two recent weekends, the same lineup of musicians in my band has played the same room (the Pacific Ballroom at the Hotel Vancouver), for crowds of similar size, at the same time of night. Yet the shows felt completely different.
We never use set lists—specific songs written in the order we would plan to play them—because over the years we've discovered that we have to read the room. Understanding the vibe of a particular crowd means having to pick tunes on the fly. Also, rather than asking organizers what they'd like us to play, we ask what kind of crowd it is: Young or old? What do they do for a living? Do they party down or tend to sit around and leave early? We can usually judge what people want to hear better than the organizers can, because, as professional entertainers, that's our job.
In fact, usually when we arrive onstage we're not sure what we're going to play until the lights go up. Often, the first thing we say off-microphone to one another is, "So, what are we going to start with?" Last night, we began with a pretty mellow bunch of tunes: "Moondance," "Fly Me to the Moon," and "Yesterday." But by then people were dancing, so we kicked into some more rock material quite quickly, and it took off from there.
On the other hand, two weeks earlier, the audience never really got that into it; even though they later said they loved our show, most of them enjoyed it by watching it, not by dancing so that they and we could feed off each other's energy.
Mark (a.k.a. "Bumpy Neurotic"), who played guitar and piano with us for both shows, thinks that being able to read a crowd comes from years of performing in pubs, bars, and small venues, all over hill and yon, where each of us had to read the crowd to survive as a musician. He has a good point. Paying your dues, in the rock band circuit, really means learning.