Perception is not reality, but perception matters most.
When in a public place, a bus, an airplane, et cetera:
Think twice about approaching a woman who doesn't know you. Then think about it a third time. Then don't do it.
Why? Even if you only want to introduce yourself or make light conversation, such innocuous advances may be perceived as a threat. It's better just to leave her alone.
Especially leave her alone if she has earphones on and is doing her best to ignore the world around her. That's a giant Do Not Disturb sign, gentlemen. Respect it.
(NOTE: The section above was written for men. The rules are different for women. I don't know why, and I don't care why. Perception is what matters most.)
When in an important meeting:
Don't take notes on your phone. Don't do research relevant to the meeting on your phone, unless you first announce that that's what you're doing - and then make it brief. Take notes using a pen and a paper notepad.
Why? In today's world, people perceive you with your head down, working on your phone, as "playing with your phone". It doesn't matter what you're really doing. The perception is what counts.
On the other hand, if you take notes with a pen and a paper notepad, people perceive you as paying attention and taking things seriously. Just accept the fact that you'll just have to transcribe the notes later.
When in a job interview:
It's good to be prepared, and it's good to be confident. But watch what you say. In fact, it's good practice to say as little as possible. Demonstrate that you're qualified for the job, but don't get carried away.
Why? The interviewers may perceive your your display of preparation and confidence as arrogance. Don't overdo it, and never, never, show all of your cards.
