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.
In other shows, usually based on a shared workplace, characters refer to each other by surnames only. 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.)
At times the last-name basis becomes jarring. (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.
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.
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- Averted in the 7th Son podcast novel trilogy, as 7 of the main heroes AND the Big Bad all have the same last name. Played straight though with the supporting heroes, who are almost always referred to as Hill and Kleinman; their last names.
- 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.
- Dawn of a New Age: Oldport Blues:
- As a result of his upper-class upbringing, Benedict refers to the other characters exclusively by their last names. Finn also does so, but to a lesser extent, using first names for people that he's amicable with.
- Sarah Travers, being a government agent, is almost exclusively referred to by her last name.
- Depending on who's talking to who, Open Blue falls in between Last-Name Basis and address by rank for military characters, First-Name Basis for civilians and/or pirates, or some mix of such. Even the addressing in narration differs between RPers.
- Most of the terrorists in Survival of the Fittest are only ever referred to by their surnames. (Danya, McLocke, Kaige, Rice, Grossi, Garnett, Konrad, Chevalier, Hurst, Richards, Baines) Dorian is the sole exception.
- Among the students, we have Anna Chase of v4, who prefers to have people use her last name Chasewhen referring to her. Liam "Brook" Brooks (also v4) is a minor variation, but probably still fits. Occasionally other students get this treatment as well (sometimes J.R. Rizzolo is referred to by his last name Rizzlo or "Riz", for example).
- In This Is War Logan specifically tells Tex to refer to him by his surname.