"Intonation is a social skill."
I posted this quote on my Facebook Page a couple of days ago and it got over 100 likes. I believe I first heard this said by Rex Martin but I'm not sure. Playing in tune with others has just as much to do with social skills as it does with the length of your instrument.
We have all played with "that guy" who thinks he has a pretty incredible ear and yet always seems to have trouble playing in tune with others. Sometimes "that guy" blames others with their words and other times they simply convey their disappointment with those around them through their body language, eye rolls or any of a plethora of non-verbal communications. No matter how great that player is, no one ever wants to play with "that guy."
You have to be flexible with your intonation always in all situations. 100% of the time. No exceptions. You can have a PHD in intonation and if you are "in tune" and the other four members of a quintet are all equally "sharp" you've got a problem. No audience member would ever hear you as in tune and the others as all sharp. You are flat. End of story.
The best set of ears I've ever played with belong to a trumpet player and my former colleague in Boston Brass, Rich Kelley. I describe him as having "beyond perfect pitch." He is blessed (cursed?) with the ability to exactly identify whether any note is sharp, flat or in tune and by exactly how much. Every single time. I know he is not unique in this regard but he is as good as I've ever seen.
Coincidentally, playing in tune with Rich is easier than with anyone I've ever played with. And that's not because he tries to steer the intonation ship from the top of an ensemble. He agrees with Pythagoras on this one and listens down. It's because he has one goal and one goal only: for the music to sound in tune. He is incredibly helpful with rehearsing and being able to identify immediately whether a player is sharp or flat in any given chord. But in the moment, he will do whatever it takes to make a chord sound in tune, which is the only goal any of us should ever have.
A very important part of playing in tune is also playing well with others.