I learned one of the most important lessons about writing dialogue and character by watching The Golden Girls.
The lesson: A character should be so clearly realized that when they say something in a scene, the dialogue should map exclusively, and exquisitely, to them. Someone else could deliver the same narrative information, but never in the same unique way this character would.
Case in point, which I remember seeing when I was a youngster: No one but Blanche could've said this.