How do you use italicized words correctly?

Italicized Words are correctly used for emphasis of a certain Word or phrase within a sentence, or a certain sentence withing a paragraph, or to denote difference in speech, or to denote thoughts rather than speech, or when writing about a song or book or the like.

This sentence does not have italics. This sentence, on the other hand, does.

The only way the above sentences are different is that one has italics.

"I like pie," Mary said. But I don't really like pie...she thought afterward.

"I hate pie!!" screamed Mary, revealing the truth.

Right now I'm listening to the song Trouble, by Coldplay.