Commas save lives

When you're talking to someone, always put a comma between what you say and that person's name. This shows that you're talking to them rather than them being a part of the sentence. Observe:

You look like a cheap French harlot, Amy. 


See how we have the comma to offset "Amy"? This is because we don't necessarily need her name in there for this sentence to make sentence. We could remove her name without changing the meaning of the sentence. It makes it less clear who looks like a cheap French harlot, but the meaning is still the same.

Here's another:

You're a mean one, Mr. Grinch.


Again, we could take out his name, and the sentence would not change in meaning. The comma is there to signal that we are talking to someone and that Mr. Grinch is not actually part of the sentence.

Here's why you need this:

Hugo: Is this your favorite luncheon meat?
Vorelli: It is Hugo.


Because there's no comma before "Hugo," we have to assume that he's part of the sentence, not that Vorelli is talking to Hugo. Instead of saying, "It is," and confirming that this is indeed his favorite luncheon meat, this indicates that Vorelli's favorite luncheon meat is Hugo. We can all see how this makes no sense. His favorite luncheon meat could be ham or bologna, but it's probably not Hugo.

Of course, there's also this classic example of the difference that can be made by one simple comma:

Let's eat, Grandpa!
Let's eat Grandpa!