Hiya! Thanks for your question! Characters’ goals and wants are a key driving point for any story.
Even if your characters accidentally “stumble into the plot”, their wants and needs still need to be related to the plot. Characters chasing after what they want is what drives conflict. Take some time to learn more about your characters. Maybe all they want is for things to return to normal. If that’s the case, then everything they do, they should be doing because it makes them feel like they are getting closer to that goal. Keep in mind that goals can change as the story progresses, and usually will.
Characters can also have small goals as well that aren’t necessarily plot-related, but they can add more depth to the person. For instance, maybe one of your characters mentions that they want to be an artist one day. Nothing they do in the story necessarily gets them closer to that goal, but it tells the reader that this character is creative, plans ahead, and has aspirations of self-improvement.
Characters suddenly finding themselves in the middle of some great adventure happens frequently in books. Take the Percy Jackson series for example, specifically the first book (slight spoiler warning here). The novel starts with Percy wanting an easy, stable life. Until one day, he gets attacked by a monster. His new goal is to be safe and survive. So now he must enlist at Camp Half-Blood where his mom gets kidnapped. Once again, his goal changes. Now, all he wants is to get his mom back, and the story progresses from there.
Ultimately, the best thing you can do is take some time to brainstorm goal ideas for your characters.
Thanks again for your question! If you need help with anything else writing-related, feel free to send in another ask. Happy writing!
– Mod Kellie
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