Hey! I can do That!
When minor characters just so happen to have the correct skill set (never seen before now) to resolve the conflict and further the plot.
Conflict! The main character is at a loss for what to do! All the other main characters are either occupied, are not present, or are equally stumped, except for the minor character who was just along for the ride until this moment. These normally unimportant characters suddenly become useful in an instant. They have a flash a brilliance or suddenly become courageous or just happen to know how to fix this one thing, etc. And then the story can go on. These characters that normally play a minor role (but still have a place in the plot to be important enough to keep around) will solve a conflict with some sort of skill or talent that was never discussed when the character was introduced (or was briefly mentioned). These characters may vary, but usually they are the ones that make things harder for the main cast because they are weaker, younger, less experienced, or just continually fumbling around getting into trouble. This works because, as a minor character, no one really knows what specific skill set they have. So when, one day, they just so happen to have this skill or talent that is useful in the moment and pushes the story along, nobody thinks twice about it. Sometimes this sudden usefulness is used to balance out all the fumbling around they do so they don't become more trouble than they are worth, though this is not always the case. These minor characters tend to be a wild card, or ace in the hole, whenever writers are unsure of how to resolve a conflict they created. After which a few things can happen. They can fade back into the into background, never to be as useful again. They can continue to fade and become completely non-existent as they are no longer useful. If this happens, sometimes they make the ultimate sacrifice and the episode can become a send off to this character. Other times, when minor characters are leftovers from other plots explored earlier, they will get recycled into this trope for no other reason than the fact that they now exist in the story. When this happens, the I-am-suddenly-useful moments are unusually downgraded. In other words, the minor character might say something like "I have such-and-such a background and can take care of said-issue." and that will be it; you won't see them doing that thing, just consider it done. This happens in "Transformers Armada". Sideswipe was a character that helped to drive the plot in an earlier episode, then in later episodes he turns out to be adept at code breaking and deciphers Decepitcon data. Nobody else has the skills to do that, so the minor character is called in to save the plot because his skills have not yet been established. On the other hand, if in the moment they made a drastic character change (eg, from being a skidish to a brave character), they may keep solving problems in future adventures and continue to grow and change, even upgrading to become a main character. (See also Breaking Out and Ascended Extra. Compare This Looks Like a Job for Aquaman. (Fixer Sue?) <- Exclusivelly fanfiction (not a trope) Examples: Anime Film
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