First Law of Metafictional Thermodynamics

"For any fictional system, the sum of the mass in the system and the energy in the system is a constant."

This is self-evident for SitComs, where all changes are reversed by the Reset Button. But it applies to systems without the Reset Button, too.

If a character or setting is added, then the plot will slow down so that the character or setting can be properly introduced, other characters or settings will fall Out of Focus, if not outright Put on a Bus. How long it takes to bring the plot up to speed, remember neglected characters, or otherwise regain a status quo depends on how much detail is added at a time; widely-spaced revelations can be worked in more quickly than large infodumps. Adding characters can also lead to Flanderization of existing characters.

If a character or setting is removed, then the plot will move faster or grow more complex to accommodate the development, or the remaining characters and settings will get more focus, or other characters will replace the ones that were removed to fill their niches. Adding energy by removing characters increases the possibilities for those who remain. Usually, some of those possibilities will crystallize quickly.

If a work gets Cerebus Syndrome, the work grows more massive, and energy must be removed. If the original work didn't have that much energy to start with, the transition to drama is likely to fail.

If a character is Flanderized or a work is converted to The Theme Park Version, the work becomes less massive, and more energy will be released into the system. This is why Flanderization and theme park versions are so widespread.

Strictly enforcing continuity adds mass as the amount of continuity increases. RetCons remove mass and release energy. This is why DC Comics is fond of Cosmic Ret Cons.

Soap Operas live on this trope; the more subplots and characters involved in them, the slower any of them advance.

This law is the force behind these tropes:
 * Suspiciously Similar Substitute
 * Sudden Sequel Death Syndrome (so the love story in the sequel can be the same as the original)
 * Pseudo Crisis
 * Economy Cast
 * And You Were There
 * Cartwright Curse
 * It's Not You, It's My Enemies (to keep UST tense)
 * Weirdness Censor
 * Kudzu Plot
 * Throwaway Country
 * The Law of Conservation of Detail

Anime & Manga

 * Bleach is a textbook case of the results of suddenly introducing Loads and Loads of Characters since the start of the Soul Society arc: the plot slowed down as tons of secondary characters were briefly introduced, and in the Fake Karakura Town arc the plots slows down even more as many of these characters are given multi-chapter fights.

Comic Books

 * This law explains the rotating roster on most Super Teams, as well as the immediate replacement of Robins and Batgirls.

Film

 * In Serenity, things pick up speed after, and really hurtle towards a conclusion after.

Live Action TV

 * Buffy the Vampire Slayer: Dawn is introduced, an episode or two are spent explaining her relationship with the rest of the group. Also when Cordelia left the show, Anya was brought back to fill the "Xander's Love Interest/Deadpan Snarker" role.
 * Original Doctor Who, which had 2-3 companions at any given time, moved languorously, frequently taking several episodes to resolve any given storyline; New Doctor Who usually only has the Doctor and 1 companion (2 in series 6), not including brief guests, as most plots are finished in a single episode.
 * New Laverne's appearance in Scrubs.
 * Although very little emotional or character development occured in Arrested Development, Executive Meddling blamed the mountains of Continuity Nods for alienating new viewers. It's one reason that show was canceled, but also one reason fans loved it.
 * In Stargate SG-1, when they were considering adding Vala to the team, one objection is that with Vala there'd be five members of the team. It seemed a good objection until someone asked why that was a bad thing.
 * This was noted by O'Neill in season 6, after . "Who decided every team that goes through the Gate has to have four people?"
 * Generally averted in Power Rangers, where the plots don't get going usually until the cast has been expanded from 3-5 rangers+allies+villains to 6-8 rangers+more allies+more villains. Played straight in RPM, where we get more plot than we have in years, after 99% of humanity has been destroyed.
 * Heroes does this to no end. Any time a new character is introduced, an old one leaves. Usually one of the women. Often in an example of Put on a Bus or Stuffed in The Fridge.
 * Seems to be utterly averted in Fringe—as more characters (from the Alternate Universe) are added and the Arc becomes more dominant, the plot speeds up and there are events of major significance per episode; thus, a simultaneous increase in mass and energy.

Web Comics

 * Acknowledged in Homestuck and avoided. Many characters are introduced slowly until Act 5, which introduced an avalanche of characters at once, which rendered many of the original characters Out of Focus. However, many characters, new and old, were Killed Off for Real during Act 5, and according to Word of God this was preplanned. However, Act 6 avoids this, despite also adding many new characters and still manages to bring characters previously Out of Focus back to prominence.