"As You Know, Bob." -- Exposition Masquerading as Dialogue
If your dialogue sounds too stilted, you may have exposition passing itself off as dialogue. Dialogue's one and only purpose is to elucidate tension between characters. It is not, ever, ever, to convey information. A bad example of what I mean:
Exposition masquerading as dialogue: "As you know, Bob, we've been stuck on this desert island for twenty years, eating only the coconuts that grow on the one tree and fish which we catch with our hands. We have several vitamin deficiencies, and you've been picking your nose this whole time. Stop it, or I'm going to kill you!"
Dramatized exposition, and one line of dialogue: Ted pounded the coconut open with a rock. It wasn't quite ripe yet, but he was so tired of fish, and his fingernails stung in the salt water where they cracked and peeled.
Bob sat on the beach a few yards away. He was picking his nose again. Again.