Your story needs a good straight man (or woman) to bring your comedic, heroic, or otherwise unconventional characters into sharper focus.
"Said" is usually the best solution when attributing a statement to a character, but there are times in your book or short story where a more specific qualifier would do a lot to paint a complete picture.
Power moments in your story set up life-changing events and propel your story forward. Writing great dialogue often depends on the subtext you create in setting up these moments.
One way to envelop your reader in a new world is to create (or appropriate) slang. Done clumsily, it can detract from your story, but Buffy and Firefly show how slang can add nuance to your story's universe.