Q. When do I capitalize in written dialog?

If the quote ends with an exclaimation or a question, that's all you need. No comma, no period. If the quote is followed by a speech-verb tag, the first word of the tag is not capitalized (unless it's a proper name or the word "I").

"I'm not an alien, you bastard!" she screamed.

But if the tag doesn't have a speech-verb, it's considered a separate sentence and you have to captialize the first letter:

"Am I an alien?" She started crying.

So far, so good. But here's where it gets tricky: If the tag interrupts a complete sentence, use commas to show that the quote isn't finished yet:

"I'm not an alien," she said, "and I wish you'd quit saying that."
(Notice the "and" is lower-case, not capital).

If the tag separates two complete sentences, use a capital letter to start each sentence and a period to indicate which sentence the tag goes with:

"I'm not an alien," she said. "Sometimes you are so rude."

Source: East Carolina University