Stop imposing my annoying habit on the person - by ceasing all interaction with that person. (Yes, I might work on ceasing the annoying habit if I think about it, but that person is probably out of my social circle in any case.) My feeling is that a person only tells you those kinds of things if they do not value you in all other ways to make up for the annoying habit. If a person loves you, they love you quirks and annoying habits and all. That's my viewpoint - from afar.

Unless I am actually harming somebody, the annoyance is their problem, not mine!

Depends on the habit and how much I’m around that person. My worst habit is I twill my hair around my fingers when I’m bored or nervous.

Depends on who that is. I am a bundle of annoying habits and tics, and my sister hates every one of them. She'll cringe, tell me to stop, and even cover her eyes at times. But since she just recently lost her husband, I stay quite about it. In the greater scheme of our lives together, this is something that isn't worth getting too worked up over.

If this someone turns out to be just some person next to me on the plane, I'm likely to suggest they put their headphones on.

It would depend on who the person was and what the habit was. If it was someone I cared about, then I would probably consider their opinion.