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Source: https://writers.stackexchange.com/a/48130
License name: CC BY-SA 4.0
License URL: https://creativecommons.org/licenses/by-sa/4.0/

Source: https://writers.stackexchange.com/a/48130
License name: CC BY-SA 4.0
License URL: https://creativecommons.org/licenses/by-sa/4.0/

@Amadeus mentions constraints. Constraints are like the walls of a house - they are limits, but also supports of the structure. The constraints define the shape of the story you tell.

If change is forced on the character by some outside circumstance, such as time, your story must perforce deal with the inconvenience of having to transform whether one wants to or not, as well as, perhaps, being unable to transform when it would have been convenient.

If transformation is always available to the character, one can examine the price of transforming - whether physiological (as @Amadeus suggests), social, or perhaps mental. (For instance, if the character spends too much time in the other shape, they might struggle to transform back.)

There is also what the character transforms _to_: becoming The Hulk would be different from becoming a moth. The latter can solve interesting plot problems, but it is also vulnerable to being squashed and to flying into a flame. The Hulk is vulnerable to nothing much.

In effect, **there are three elements you want to balance: the transformation's capacity to solve problems, the ease of transformation, and the downsides of transforming.** If transforming is easy, and its capacity to solve problems is high - the downsides must be serious. If there are few downsides then either the transformation creates more problems than it solves (think of the traditional werewolves), or else the price is high. The particular trigger would be derived from this consideration.

Original score: 2