Amy: That's her name, Philip.
In many shows, the characters refer to each other by their given names, and the audience refers to these characters as such. We refer to the friends of Friends as Rachel, Phoebe, Joey, etc. This is usually the case in Dom Coms and other shows where many of the characters are related and therefore have the same last name. However, The X-Files could probably be considered the Trope Codifier, as that series took this trope to an extreme. Try thinking of a single character in that series who isn't referred to with either a descriptive title, or their last name.
In other shows, usually based on a shared workplace, characters refer to each other by surnames only. Nobody (except his mother and his ex) ever calls House by his given name, Greg; not even his best friend, whom House also calls by surname, Wilson. Most of the characters on CSI call Gil Grissom by his surname (as they do Brass, Hodges, and Ecklie). Accordingly, since this is the way the character is canonically addressed, fans will refer to them by their surname as well, sometimes to the point of forgetting a character's given name entirely.
(These characters are not referred to with titles, either. It's not "Dr. House" to the other regulars. It's just "House.")
At times the Last-Name Basis becomes jarring. When House's Wilson began dating Amber (the only first-namer on the show,) she still referred to him as Wilson, possibly because the writers were so used to the name they just didn't think about it, and possibly because they thought the viewers might not know who "James" was. (This kind of situation may be used to set up a joke if the character has an embarrassing first name.)
Characters on a last-name basis are much more likely to be male than female. Sometimes there's a Double Standard for this trope: the same show may refer to men by their last names and women by their first names. In a few of Dan Brown's books, regardless of how the characters address each other, the narrator mostly calls men by their last name, and women by their first name, including protagonists or co-protagonists.
If only some characters in a work get this trope, it is frequently because they have a boring or common first name (like John), or an embarrassing or unusual one. Switching from Last-Name Basis to First-Name Basis may indicate a Relationship Upgrade, platonic or otherwise.
How much Truth in Television this is varies based on time period and cultures worldwide.
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- Real Life
- In This Is War Logan specifically tells Tex to refer to him by his surname.
- In Critical Hit, a live play Dungeons & Dragons podcast, Ket refers to his teammates almost exclusively by their last name while giving orders in battle.
- Everyone in Wolf 359 refers to one another by their last names, with the exception of very emotionally heavy situations. This often leads to many characters only being known by their last names for a long time (Minkowski's first name - Reneé - wasn't revealed until the start of season 2).
- Averted with Cutter, who makes a point of referring to people by their first names.