In general when in passenger facing situations at, for example, a ticket desk, I found a good approach was to let them talk until they run out of things to say, whilst listening and making the odd note, which will often make them a little more circumspect about what they say. Then ask them what they would like us to do about the situation and see how far we can meet their demands. Very often their demand is less than the company might have offered, in which case it is easy to exceed their expectation.
On the other in situations when I am not representing, or accountable to, another entity I will give them the benefit of what I really think!
I also often remind myself when people are rude, bullying, aggressive and unreasonable that they are they to allow me to prove my superiority over them. Arrogant in the extreme, but it works.