Newscaster: ...Turanga Leela.Bender: Philip?!
Amy: That's her name, Philip.
—Futurama, "The Problem With Popplers
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 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. Nobody ever calls House by his given name, Greg, including his best friend, whom he calls Wilson. Most of the characters on CSI call Grissom by his surname, as they do Brass, Hodges, and Ecklie. Accordingly, fans refer to such characters by their singular surnames, 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 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.)
Shows which use this trope can instill an artificial dislike for a new or guest character in the audience by putting them on a first name basis with the regular cast. They seem out of place as a result, which causes viewers to regard them as "bad" despite there being absolutely nothing to fault them for.
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.
Last-Name Basis is Truth in Television for many non-US cultures; in Latin America it's not uncommon to see close friends calling each other by their last names. In Japan, it is standard for friends to speak on a Surname Basis; Given-Name Basis is a more significant trope.
Also Truth in Television for shows set in historical times (before about 1945 in North America and 1980 in the UK). In Regency England, for instance, first names were only used by adults when addressing children (and parents when addressing their own children, even if they were adults), and among siblings or very close female friends. Husbands and wives only addressed each other by their first names when alone: in public or even amongst their family, they often referred to each other more formally. In many ways, the use of the first name became the English-language version of the French tutoyer, as if addressing someone by their first name without a good reason showed that you didn't see them as your equal. While modern Americans see using the first name as friendly and egalitarian, someone from this time frame would see it as pushy, rude, and intrusive.
How much Truth in Television this is for modern US culture varies, especially for professional settings. Title-and-last-name-basis is often used in formal business relationships, for example with a customer or a boss. Even in less formal settings, Last-Name Basis is seen among groups of coworkers, especially when there is a lot of first-name overlap. It's also the standard in the US military, where you might be a bit hazy on the first name of your best buds.
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