It depends on the level of formality. If you're friends with Nicole, you can address her as Nicole. If she's a stranger, you can address her as Miss Wheels, or Madam Wheels, depending on her marital status. You could still call her Nicole, though, it all depends on the situation or where you're meeting each other, and she could specifically request you to call her Nicole. We usually go with first names for celebrities (Emma Watson, Michael Jackson), unless you're reporting about them, or writing their bio, etc. So if you do meet Emma Watson (I would be extremely jealous), and you're not from the media, you can call her Emma. But if you're updating her wikipedia page or writing about her latest movie, you usually use her last name. For politicians, usually last names. Trump, Bush, Trudeau, Macron, etc. There may be exceptions. It gets complicated with Prince Harry. Prince is not his first name, it's his title. I'm not British, so I'm not precisely sure what they use to address him, but usually with royalty, people use titles like "your highness", etc.

It really depends on the speaker's relationship with the person being referenced is... Public figure (e.g. Donald Trump) - then just their surname as you mention is fine (if it's distinctive enough), but often their full name Stranger - usually their full name Slightly acquainted/professional superior (large company as small companies tend to be less detached in social interactions) - Mr/Ms/professional title + surname Well acquainted/friends - forename In the cases you provide: Ms Wheels/Nicole (depending on how well you know her); and likewise with Susan Wang (Susan/Ms Wang). In both cases, I say 'Ms', as it's more neutral and there's no need to 'guess' whether they're married on not (i.e. wondering whether to say Miss or Mrs). As for Prince Harry... uhm... presumably you're not well acquainted with him, but when nobility/royalty is concerned, convention would suggest that you use their noble/royal 'style' until given permission/instruction to address them otherwise. In his case: Your Royal Highness when addressing him directly; and His Royal Highness, Prince Henry if speaking to someone else while in his presence.

We call each other by our first names. My name is Ellis W - so you wouldn't call me W as we have neither established that it's okay for you to do that, and nor am I a person who refers to themselves by their surname. Surnames don't have much significance here as we don't really call them 'family names'. With some names that are more common, as per your Trump example, we call him that because it is a distinctive name and everybody knows who you are referring to. A lot of celebrities get called by their first and last name. Emma Watson, George Clooney, Meryl Streep.