The first point is an imperative one. As musicians, we are taught to share our (musical) opinions all the time. Sometimes it can be challenging to not let that naturally extend to things off of the horn. I was taught to ask myself three questions any time I want to open my mouth to criticize anyone or anything:
1. Does this need to be said?
2. Does this need to be said by me?
3. Does this need to be said by me right now?
Unless I answer yes to all three of those questions, I've learned to keep my mouth shut.
John's point was that he didn't criticize colleagues. He didn't criticize conductors. He didn't complain about the pay on a gig (which he had already agreed to or he wouldn't be there in the first place!) He kept his mouth shut as a sub and kept his head down.
And the second point will get you hired over and over again. As Rex Martin used to preach to us at Northwestern, our job as musicians is to make those around us sound better than they actually are. And John shared a compliment that Woody English, the fantastic former trumpet player for the Army Band, once gave to him: