Fan Casting First-Name Basis in Sort by Tropes

The sudden switch in nickname or honorific when referring to a character, usually from their family name to their given name, is frequently used to demonstrate a significant change in the relationship between two characters, the desire for one by one party, that they have evaded listeners who require more formality, or that they are Not Themselves. Never using any formal indicators and just calling a character by their exact name is about as common to show the same thing.