How do you indicate internal thought vs. dialogue? (with examples from Stargate SG-1, Heroes, and Supernatural)
Unlike some of the questions we answer here, this is an issue of style, and there is no simple right or wrong answer. There are, however, common conventions you can follow.
The dialogue part is easy: all dialogue should should have quotation marks at the beginning and end of the spoken sections, with the punctuation on the inside.
"Jack," Daniel said, exasperated. "They're a very special culture."
"Save the cheerleader, save the world," the man said, and then suddenly the subway car was full of sound.
[If you're interested in more specifics about dialogue, take a peek at this post and keep an eye out for a feature, Dialogue: the Good, the Bad, and the Ugly coming up in the next few months.]
Thought, however, is a little more complicated. The first issue is how the statements are being presented:
Not this time, Sam wanted to say, I didn't wait for him to attack. It was murder. "You're right," he said instead. "I did what I had to do."
Sam wanted to say that Dean was wrong, that he didn't wait for the attack. It was murder. "You're right," he said instead. "I did what I had to do."
In the first example, we're seeing Sam's exact thoughts, in the form of dialogue with himself, so they need to be differentiated from the narration in some way. In the second example, Sam's thinking about something, so it's only part of the narration. When the narration is in first person or tight third person (in the specific character's head, rather than more removed), all narration is technically internal thought:
Jack waited--patiently, he thought, especially since his secretary only glared at him once--and practiced his yo-yo tricks, eyes going back and forth between the clock and the phone.
But since it's not structured like dialogue, italics aren't necessary.
[Want to know more about different narrative points of view? There'll be a Feature out addressing just that (as well as how POV and tenses affect the tone of a piece) the end of this week.]
But here's where it gets tricky. Technically, both of these are correct:
Not this time, Sam wanted to say.
'Not this time,' Sam wanted to say.
Personal preference leans towards the former, but as long as you, the writer, are consistent, it's not especially important. There are things you shouldn't do, though:
*Not this time.*
~Not this time.~
and other such variations are just plain wrong, even and especially when paired with italics. Special characters like that aren't punctuation, and really only serve to clutter your space. If you can't italicize (the formatting of the location where you're posting can be the main culprit here), use the single quotation marks. Don't abuse the Shift+digit combination.
As an aside, one of my fellow grammarians tells me that Australian standard says that dialogue goes in single quotation marks. We both suggest that if you're using that standard, italics are a must.
As another aside, another problem can be telepathic dialogue. Italics are great for that, too:
I know you can hear me, Sylar said, as loud and clear as if Matt had actually heard him. Run, little man.
Asterisks and tildes need not intrude.
Basically, the important thing is to be consistent. No matter which way you choose to go, as long as you do it that way every time, you're doing it right.