Nurse Mary: Tell me, Captain Blackadder...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, 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. Another common trope is a character actually saying the other's name rather than some rude variant of "you" or of your name to show a relationship getting closer. The other situation is a character who was previously irritated to be called by a "cutesy" name finally accepting the moniker, usually from the sort of overly cheerful character who does that with everyone's name. Propriety may demand that it be used only in private. Referring to a character by a first name to a third-party can reveal the nature of the relationship. This can range from a total accident to an intentional flaunting. Juniors may stubbornly insist on titles and family names when they think the senior is using his authority to feign friendship. For the creepy, an enemy uses these as Terms of Endangerment. Keep in mind that in Japanese the family name comes first so in anime and manga this is more of a "last name basis". That Other Wiki almost always reverses this order for Americans, as do some translations, complicating things considerably. Bonus points are awarded if you notice in the audio that one name is used but in the subtitle a different name is used. Note that this is much less important in languages other than Japanese, because most languages just use First Name Basis to denote equality and/or friendship, more or less. Except when they don't. This trope is the polar opposite of They Call Me Mister Tibbs. Compare Last Name Basis, when the characters never made the switch, Full Name Basis if they never even switch to last name, and Hey You. May get interesting when the character is Pals With Jesus. Not to be confused with One Name Only, such as Prince, or Cher.
Cpt. Blackadder: Edmund.
Nurse Mary: (grins) Edmund.