An issue that seems to confuse many authors is when to use italics, underlining, all capital letters, or boldface in their writing. Each of these has its own function, and some should be used more frequently than others.
Let's look at using these things for emphasis in a book. The quick answer to this is for the most part, you should use italics for emphasis. Boldface should only be used if a sentence is already italicized and you want to further emphasize a particular word in the sentence. All capital letters should not be used for emphasis! In fact, all capital letters don't usually have a place in a manuscript, the only exception being when you want to indicate that someone is screaming in dialogue.
Similarly, underlining really has no place in a manuscript. Yes, I know back in elementary school our teachers told us to underline book titles, for emphasis, etc., when writing, but that is only because you can't italicize when you're physically writing on paper. If you think you want to underline something, it should probably be italicized.
And this brings me to another point. Don't overuse italics. In fiction writing particularly, I often see authors italicize large sections of text to indicate a flashback or something similar. The problem with this is that italics are hard to read and so most publishers prefer that they be used sparingly. A better way to indicate a flashback or dream sequence is to use a scene break. The same rule applies to using italics for emphasis. If you're emphasizing words in every other sentence, the impact of the italics are lessened.
Many publishers also prefer that italics be used when a character is thinking something (e.g., I really like ice cream, he thought). You will probably be safe if you indicate internal dialogue or thoughts in this way, but this is not necessarily consistent between publishers and may be changed when a publisher's internal copyeditor looks at your work.
Stay tuned for part two, where we'll discuss when to italicize titles of things and when to put them in quotation marks.