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.
If the "Over" character is introduced later, this is Replacement Flat Character.
- 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, but he tries to keep some aspects (such as the sexual fetishization of manga characters) on the DL. 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. For his part, Madarame is introverted and socially awkward, but functional enough to form several deep friendships. Kuchiki, on the other hand, is so socially dysfunctional that it occasionally borders on criminal, and his presence in the club is merely tolerated, and then only because Sasahara feels sorry enough for him to vouch for him.
- Run Fatboy Run provides a good example. Main character Dennis is a lazy, unmotivated Manchild 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 addicted to alcohol and gambling.
- Will & Grace - Jack and Will are Over and Under examples of Camp Gay, respectively. ("Over" and "Under," not "Top" and "Bottom". Get your mind out of the gutter.) The fact that people actually refer to Will as Straight Gay shows you just how effective this trope can be.
- Dharma's parents from Dharma & 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.
- 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.
- The whole cast of Coupling is designed this way. Steve and Susan are the everyman Unders, whose relationship is charted over the course of the show. They each get a friend of the same gender who is completely Over when it comes to dating confidence and promiscuity (Patrick and Jane), and another who is completely Over when it comes to neuroses and hang-ups (Jeff and Sally).
Steve: Let me explain, Patrick. Here on Earth, there is a gap between seeing someone you like and having sex with them that we like to call "conversation". In Jeff's case, it can last for up to ten years.
- On Cheers, Diane was originally the only character who was well-educated and interested in "high" culture, and was generally used as an over-the-top caricature of pretension: if she wasn't insisting the other characters abandon their "low" pursuits for something more sophisticated, she was excessively analyzing the philosophical, psychological, historical, cultural, or literary implications of every minor event. However, Season 3 introduced Frasier, who shared Diane's interests and background, but in a much more understated way. He was still prone to being verbose and analytical, but was never as mono-maniacal about it as Diane could be, and usually engaged the other characters in actual conversations, rather than monologuing as Diane was prone to do.
- Excellen Browning from Super Robot Wars Original Generation is another two-for-one example. She's usually a Hard-Drinking Party Girl who speaks of the Power of Love and has apparently chosen The Stoic 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.