A few years ago we had a mechanical issue at an outstation. A commuting 777 Captain for our mainline counterpart came storming down the aisle, demanding to know what the mx issue was. He even proceeded to interject his opinion that the write-up was trivial and we shouldn't be taking a delay. Fortunately, I was flying with a 20+ year guy who had no problem putting him in his place. Before tactfully telling him to sit down, he asked the commuter how he would react if one of us butted into his cockpit and started questioning his decisions.