Over and Under the Top

In a deliberate modification of having an Economy Cast, a series will sometimes have two characters who clearly share a given value X for a particular trait.

The 'Over' is the fanciful depiction who never has to worry about being over the top. The character can be a guilty pleasure, the comedic version, or to show the writers are self-aware about a topic without becoming a full parody. His proximity to others makes them immediately more subtle, and can make The Reveal in other characters more credible when he acts as a red herring. He is allowed to be campy. Whenever the 'Over' does X thing, we know not to take it seriously.

'Under' is the semi-moderate to normal version of X, for a given value of normal. He is needed when the writers want to do a "serious" plot that requires X trait. Him being more moderate means he has more potential for the audience to identify with him and makes him easier to project whatever traits the audience wants. His possession of X trait is also useful as it usually prevents the audience from accusing writers of outright making fun of X trait. When the Under does X trait, we know it's a dramatic X.

If a show attempts both the comedic and dramatic, this sometimes occurs in a single character, leading to the sort of person who gleefully jumps on the object of their affection while needing an entire story arc to consider mundanely kissing them.

Naturally, this all depends on the audience being smart enough to spot it; if not, one can get Misaimed Fandoms, loud complaints, and worse.

Examples of Over and Under the Top include:

Anime & Manga

Negima! uses this as a way of handwaving the appearance of a Harem. We dismiss Ayaka's apparent love for Negi as being "serious" because she goes about it in a loud, hilarious, melodramatic way. We accept Nodoka's as genuine because she does it in a subdued, shy, honest way.
Ryuichi from Gravitation is a good example of the "one-character" variety.
Ah! My Goddess. Affection for Keiichi being the X value, with Peorth (and others) being the "over" and Belldandy the "under".
Sasahara from Genshiken is an Otaku, or at least a closet case. As the focus character, he has to be at least somewhat relateable, so his otaku-ness is, at least at the start, overshadowed by the other members of the Genshiken, especially Madarame.

Film

Run Fatboy Run provides a good example. Main character Dennis is a lazy, unmotivated Man Child who left his pregnant fiancée at the altar out of panic. Her new American fiancé is successful, attentive, and everything Dennis wasn't, at the expense of being The Comically Serious with a slight competitive streak. In order to save Dennis from looking like an irredeemable loser, his best friend Gordon is a huge exaggeration of all his negative qualities, as well as being The Alcoholic and The Gambling Addict.

Live Action TV

Will and Grace - Jack and Will are Over and Under examples of Camp Gay, respectively. (I said "Over" and "Under," not "Top" and "Bottom". Get your mind out of the gutter.) The fact that people actually refer to Will as Invisible to Gaydar shows you just how effective this trope can be...
Dharma's parents from Dharma and Greg are both aging hippies, but Abby is a sensible, friendly hippie while Larry is a raving Conspiracy Theorist whose drug experimentation left him rather worse for wear.
How I Met Your Mother - One great (and rather extreme) example would be the portrayal of relationships. For the 'under', you've got Marshall & Lilly: engaged, dated for 9+ years, love each other very much. Then for the 'over' you've got Barney, who almost never sleeps with the same girl twice (often abandoning them), picks up hundreds of girls at bars using the most insane lines, believes that every aspect of his life is AWESOME. Then throw the Dogged Nice Guy Ted as a main character in the middle of this. The fifth character, Robin, falls somewhere between Ted and Barney, a serial monogamist who is wary of marriage and children.
On Top Gear, the dynamic of Jeremy Clarkson and Richard Hammond (both over) and James May (under) in their brashness and love of automotive power and speed. Clarkson or Hammond driving fast is everyday stuff. When "Captain Slow" drives fast, it's serious.
- In the car-modifying challenges, Clarkson is over - his approach can be summed up as "more power" and "hit it with a hammer", May is under - considered and technical, and Hammond is somewhere between the two.
On The Big Bang Theory, Leonard is the straight man in his group of nerds, written to be normal enough to be relatable to the average person. Sheldon is written to be ridiculously over the top, hyper-intelligent but completely unable to comprehend normal society.

Video Games

Excellen Browning from Super Robot Wars Original Generation is another two-for-one example. She's usually a Bottle Fairy who speaks of the Power of Love and has apparently chosen The Stoic Badass Kyosuke Nanbu at random as the recipient of her overexaggerated affections. In spite of this, the two have some of the most genuinely tender moments in the entire series.

Web Comics

In Darths and Droids, Annie is the "over" and Ben the "under" version of The Roleplayer.