Me being my (not) normal self, have done a variety of serenades, to a variety of people -- many of which have been strangers.
If you serenade a stranger, one of five things will happen:
1. They will smile and/or laugh (because it's refreshing).
Sometimes when you serenade a stranger, they will smile because you just made their day. They think it's great someone has the enthusiasm as such and is willing to spread a little bit of joy. These are the fun people to serenade.
2. They will laugh (because they think you're stupid... or drunk).
Serenading strangers isn't a normal thing that a normal person would do- so sometimes people automatically assume that you are intoxicated simply because one cannot be that happy. Or they will laugh because they just don't know why someone is being that dumb.
3. They will sing along.
Sometimes you just have to have a jam session with a total stranger, even if it means it has to start by you serenading them... maybe you share the same passion for the same song. This is he most fun way to serenade a person.
4. They will stare at you blankly.
Not every day does a random stranger serenade you, so yeah, they might be a little shocked and not know how to respond. Even though you keep singing, they will keep staring. This is probably the most awkward way to serenade a person.
5. They will tell you to shut up.
Some people don't enjoy (or appreciate) that someone has the amount of energy you do to start singing to them, or they're just having a bad day and don't want to hear you sing. This will result in them yelling at you to "be quiet" in nicer words.