Rule #1: Don't piss off the sound guy!
Let's start with something not so obvious. Every person that comes into a venue will affect the sound by the heat they give off and the CO2 and water vapor they breathe out. Changing the temperature and density of the air in the room can directly influence the sound, especially if the HVAC can't keep up. Does it matter? Maybe, maybe not. If the system is struggling already and then things get hot and steamy, you could be in for a long night. If things are bright and brittle, maybe a change of atmosphere is just what you want. When it's the atmosphere that's changing the highs will be the canary in the coal mine. As the sun sets on an outdoor gig and the air cools off you can find yourself with a very sharp sounding system compared to how it sounded in the hot, humid afternoon.
As to the bodies themselves, they play a part too. Filling an empty space, be it a pub or a stadium, with a capacity crowd will do a lot to dampen reverberation. Beyond that, a poorly designed system can allow people to get their heads right in front of speakers that are meant to be pushing sound right out to the back of the room. That can be dangerous for the poor soul standing right in front of a 12" mid with 1000 watts behind it and can make the engineer miserable as well.
In the end it all comes down to experience and using your ears. You can plan all you want but unless you really know a venue well you have to be ready to make adjustments as things happen. In one room with a bad seating arrangement and poor climate control you could be scrambling all night. In another, with nice raked theatre style seating and good air conditioning you may not notice much change at all once the audience loads in. Once again this pertains to the littlest coffee shop and the biggest stadium. A good engineer will keep his ears open and try to take a fresh look at how things are sounding as the night goes on, making adjustments for temperature, humidity and crowd as needed.
Which reminds me of a gig I did in a high school aud a few years ago. It was for an organization run by a member of a famous band who was to be on site that day. Just after sound check someone from the school came around to show me how the walls of the aud were set up with hinged panels that were wood on one side and cloth on the other. I had rung out the room and sound checked the band with the wood side showing so I mentioned how that was interesting and went back stage to talk to the band. When we came back out to play someone had thoughtfully turned all the panels cloth side out and there was no time to change them back. That, and the addition of four hundred kids made it a drastically different sounding start from what we had just heard at sound check. And there stood my boss, right at my elbow. I adjusted pretty quickly and he said he hadn't noticed anything wrong so it worked out well, but my heart was racing there for a couple minutes until I had everything settled.
http://smart2noise.blogspot.com/2012/09/empty-room-vs-full-of-fans.... <-- Read the original post here!