Dialog can speed up or slow down the plot. This depends on the type of dialog you write, such as lengthy talking points a character shares with another character. They tell a story in the story. Speeding up can be quick, concise, and reveal plot through conversation.
Dialog shows character How and what someone says often shows what sort of person they are. Accents, forma or informal speech, and even slang can show a character’s background or mood. You shouldn’t be too heavy with accent, however. That can be too distracting. Add just enough, and move on with the story. Try to stay in the head of the character to be consistent with the style of talking.
Giving of information. Dialog works best as a tool to reveal information, by allowing one character to ask the questions the reader might be asking themselves. Although the method can be effective, you don’t want to use it too much as an easy way to ‘tell’ instead of ‘show.
He said. She said. There’s the ongoing debate over using the word ‘said’ in dialog tags. Readers don’t even see them, half the time, and using overly dramatic tags leads to pulling the reader out of the conversation as well. I use both sparingly, using tags to indicate what character is doing along with the saying. In other words; you don’t have to use ‘said’ if you’ve made it clear who is talking, and when they are talking.
Using the tag ‘said, however, is perfectly acceptable.
Here are some links on the topic of dialog;
9 Easily Preventable Mistakes Writers Make With Dialogue. The Creative Penn
My Dialogue Sucks: Tips For Improving Dialogue In Your Novel. The Creative Penn
Top 8 Tips for Writing Dialogue by About.com
How to Punctuate Dialogue by The Editor’s Blog