Dialogue...

Looking at all my writing books, I noticed I have two books on writing dialogue.
It is tricky to write. Do I write as people speak or do I write gramaticially correct? How much internal thought do I use?

How much dialogue will I use in Left Unsaid?

Definitions are interesting to me. So I looked up dialogue:
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di·a·logue or di·a·log
Pronunciation Key (d-lôg, -lg)
n.
1. A conversation between two or more people.
2.

a. Conversation between characters in a drama or narrative.
b. The lines or passages in a script that are intended to be spoken.

3. A literary work written in the form of a conversation: the dialogues of Plato.
4. Music. A composition or passage for two or more parts, suggestive of conversational interplay.
5. An exchange of ideas or opinions: achieving constructive dialogue with all political elements.