For any fictional system, the sum of the mass in the system and the energy in the system is a constant.
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.
This law is the force behind these tropes:
- And You Were There
- Cartwright Curse
- Doomed Hometown
- Economy Cast
- Four Lines, All Waiting
- It's Not You, It's My Enemies (to keep UST tense)
- Kudzu Plot
- The Law of Conservation of Detail
- Sudden Sequel Death Syndrome (so the love story in the sequel can be the same as the original)
- Suspiciously Similar Substitute
- Pseudo Crisis
- Remember the New Guy?
- Three Lines, Some Waiting
- Throw-Away Country
- Two Lines, No Waiting
- Weirdness Censor
- 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.
- The Divine Comedy: The more sub-divisions there are in a circle of Hell or sphere of Heaven, the longer it takes to go through. This is why half of Inferno is spent in the Eight Circle with its ten sub-divisions and why Eight Sphere of Heaven takes a similar amount of time with its line-up of those who represent the theological virtues.
- David Weber's Safehold series is a good example too: in first book (one main POV) action took two years in-story. Book six (after adding many, many POVs during previous ones) in longer, but concerns only five months. As to NIE/Bnote , zig-zagged: in book two it's less than in book one, then book three has more, then books four and five see steady fall and in books six and seven NIE/B rises.
- George R.R. Martin's A Song of Ice and Fire demonstrates this trope nicely. As more and more POV characters are introduced, the plot has slowed down considerably as time and space is used to develop each POV. The first three books are very well written and gripping whereas the fourth is slower paced and focuses mostly on sideplots with hardly any of the series' main protagonists featuring. The fifth gets things back on track but is still very slow.
- Martin is very much aware of this trope. He's stated that there will be no more new POV characters in future books. And half-jokingly claims that he needs to start killing off more characters in The Winds of Winter.
- In Serenity, things pick up speed after Book dies, and really hurtle towards a conclusion after Wash dies.
- 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 Daniel had gone missing, and the Russians wanted someone on SG-1. "Who decided every team that goes through the Gate has to have four people?"
- Power Rangers: Generally averted, 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.
- Fringe: Seems to be utterly averted. 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.
- Night Court: Endemic through most of it. The strident young feminist prosecutor was replaced with another strident young feminist prosecutor. The sarcastic elderly bailiff was replaced with another sarcastic elderly bailiff (though this was averted when the show's third bailiff was the much younger Roz to avert another Actor Existence Failure).
- 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.