[[quoteright:240:[[VisualPun http://static.tvtropes.org/pmwiki/pub/images/weasel_55.jpg]]]]
[[caption-width-right:240:This is not a Necessary Weasel. It's a Gratuitous {{Weasel|Mascot}}, courtesy of the RuleOfCute.]]

->''"Be happy that weasels infest the world. Weasels are like motor oil for society. It wouldn't be fair to judge motor oil outside the context of an engine. If you put motor oil in your mouth, it would be filthy and slimy and leave a bad taste. But when that oil is inside an engine, it does an important job and you're glad it's there. Weasels are the same way. Slimy and disgusting, but essential."''
-->-- '''[[ComicStrip/{{Dilbert}} Scott Adams]]''', ''The Way of the Weasel''

As a logical extension of TropesAreNotBad, many tropes that might otherwise come across as gratuitous, offensive or just plain wrong in most genres are considered not just accepted in certain genres, but are practically a part of the genre. Complaining about the simple use of the tropes (as opposed to particularly offensive variations) in said genres is rather short-sighted and pointless, since, well, it's in almost ''every'' other work in the genre.

Can often be the cause of an EnforcedTrope. Some AcceptableBreaksFromReality are Necessary Weasels to particular genres as well. Contrast with the BadWritingIndex, an article listing some real (versus merely perceived) flaws.

!!Artistic "flaws":
* RuleOfCool in general; "realistic" does not always equal "interesting."
* MoreDakka, StuffBlowingUp in summer action films.
* Blatant WishFulfillment and MarySue characters in {{Fairy Tale}}s and mythology.
* {{Invincible Hero}}es in combat-oriented {{video game}}s. [[NintendoHard Depending on the difficulty at least]] (or the player's skill).
* ToiletHumor in comedies featuring babies or animals.
* An AudienceSurrogate character in a [[HaremGenre Harem]] anime.
* AnticlimaxBoss in WideOpenSandbox games. If a boss must be fought, and players can be optimized for something other than combat, then the boss must be beatable by the weakest character who can reach the fight.
* BossArenaIdiocy because they would be frustratingly hard or {{Unwinnable}} otherwise.
* PurpleIsTheNewBlack, and any other specific attributions of a particular colour to [[LightEmUp light]] or [[CastingAShadow darkness]] (like [[HeavenlyBlue blue]] or [[RedAndBlackAndEvilAllOver red]]).
* TheGodsMustBeLazy and similar tropes, like GodsHandsAreTied and HaveYouSeenMyGod in settings where vastly powerful deitie(s) exist. Such tropes preserve {{conflict}} by keeping the story from favoring the good side too much by making it too powerful, and allow for a [[MostWritersAreHuman human]] underdog hero to step in when the god(s) cannot or won't.
* WillTheyOrWontThey in stories involving the main character picking a LoveInterest. Unless there's something else to generate conflict, this trope will be used to keep the story from ending until TheyDo.
* CommonHollywoodSexTraits: Because ''real'' sex isn't always sexy, and a realistic depiction would likely go into IkeaErotica territory, most stories that feature sex or its aftermath are going to involve IdealizedSex.
* TheCoconutEffect: A sound effect, color, etc., that people expect to be there, such as horses' hooves that sound like coconuts being smacked together.

* FasterThanLightTravel: usually present in space-based ScienceFiction and virtually all SpaceOpera. Without it, you can't really go anywhere other than the Solar System, which aside from Earth is made up of some fairly unattractive real estate.
** AliensSpeakingEnglish or EternalEnglish, whenever the plot is not about learning to communicate with them. Sometimes hand-waved with a [[TranslatorMicrobes Universal Translator device]], but either way, the protagonists often need to communicate, and going back and forth with an interpreter is too clumsy to do all the time.
** ArtificialGravity, for "[[MohsScaleOfScifiHardness soft]]" ScienceFiction works (though otherwise "hard" film and especially television ScienceFiction will sometimes make use of it due to the extreme difficulty of producing true "zero-g" on camera).[[note]]Although, this may depend on how it's presented and achieved in-story. For example, just spinning the ship and having people walk on the side walls is perfectly reasonable.[[/note]]
** InertialDampening: Often accompanies FasterThanLightTravel (or at least any acceleration/deceleration that can be expressed as ".C"), needed to keep the crew from being rendered a fine paste in high-speed maneuvering or entering/exiting FTL travel.
* NoConservationOfEnergy in portrayals of StockSuperPowers.
* Ignoring of the SquareCubeLaw (as well as other laws of physics) in SuperHero, HumongousMecha, and AttackOfTheFiftyFootWhatever stories, as well as some types of StarfishAliens.
* InstantExpert in {{Role Playing Game}}s.
* BenevolentArchitecture - Sure, there is very slim chance that video game character's surroundings will be perfectly suited to that character's unique powers or abilities. Or that wreckage will form a path, rather than an obstacle. However, being realistic would mean {{Unwinnable}} or else easily passed.
* ThePerryMasonMethod: Because once the audience knows who the real culprit is, there's usually no real reason to continue the story.
* {{Painfully Slow Projectile}}s and HitboxDissonance in BulletHell games -- otherwise they'd be impossible rather than just NintendoHard.
* JumpPhysics in {{platform game}}s.
* PowerCreepPowerSeep in {{Crossover}} {{Video Game}}s.
* The {{Masquerade}} in UrbanFantasy and other contemporary fiction with fantastic elements, allowing the writer to add such elements to the story while [[PlausibleDeniability keeping the setting relatable for the audience]].
* Similarly, ReedRichardsIsUseless is necessary to keep the larger setting of a superhero world from transforming into ScienceFiction with leotards.
* ContrivedCoincidence in {{farce}}. The more things that go humorously wrong, the better, so plausibility can go out the window.
* CellPhonesAreUseless in a ClosedCircle.
* ImprobablyFemaleCast in MagicalGirl shows.
* ImperialStormtrooperMarksmanshipAcademy - If the antagonists hit their mark, the story would be over.
* MilitariesAreUseless in a ZombieApocalypse because otherwise it wouldn't be TheEndOfTheWorldAsWeKnowIt. Can sometimes be avoided however, if the story takes place early on or in a small area, allowing the military to save the day in the end ([[InferredHolocaust long term implications be damned]]); or the zombies are [[OurZombiesAreDifferent more powerful than usual]].
* OrcusOnHisThrone: Sure, it would be sensible for the BigBad to personally hunt down TheChosenOne. But then the protagonist would get splattered and there'd be no plot. Or the BigBad gets splattered too early and the plot is over.
* TheMainCharactersDoEverything because otherwise you'd have a massive cast fighting for what little spotlight there is. (And in the case of live-action, massively cost-prohibitive.)
* Acting TooDumbToLive (even for a single thing, for one brief moment) on horror stories. After all, even if DeathByGenreSavviness or [[DeathByPragmatism Pragmatism]] exists, many horror films depend on the heroes-slash-victims doing that one stupid thing that gets the ball rolling like [[DontGoIntoTheWoods going camping into the deep dark woods]] or visiting that friend on LovecraftCountry or [[CuriosityKilledTheCast getting curious about that weird thing they just noticed]].
* AllWomenAreLustful or AllWomenArePrudes: Guess which one of these two opposite tropes is enforced in the porn genre. You have a 1:1 chance.
* YouCantThwartStageOne because even if by all rights they should have be able to, doing so would usually make the entire rest of the story [[{{Padding}} irrelevant to the now resolved plot]].