Our story has a plot, even if we keep it simple such as two people meeting and falling in love. Problems arise when out characters conspicuously do things purely for plot purposes rather than for ones that fit the characters. The most obvious one is the damsel-in-distress whose only purpose in the story is to be rescued by the male hero, she has no agency beyond that, she doesn’t learn, and often gets into the same trouble all the time. The damsel-in-distress isn’t a plot puppet because she needs rescuing, everyone needs help sometimes, it’s because that’s all she does.
Plot puppetry doesn’t only arise in one dimensional characters who only exist to do one thing over and over, it can appear in our fully rounded characters too. Arguably it’s even more problematic in well-rounded characters because it’s so conspicuous that what they’re doing is out of character and purely for plot purposes. These can be big things like the capable female detective who suddenly becomes a damsel-in-distress when she walks into the obvious trap without even considering how obvious it is. To the character who keeps saying they don’t like weapons and then when someone uses one just shrugs it off. It’s also important to note that there’s a difference between charging in because they’re the sort of character to charge in without considering their own safety and a character charging in so they can then be rescued by another character or to advance the plot.
An important question is: Why are they doing this? Would it make more sense for them to do it a different way? Or would it make more sense for another character to do it? Are they doing it to make another character look good? If so, why?
We can have characters so things that appear out of character but they should have a reason for doing it. The classic would be the ‘fake date’ plot in some romances. These two (or more) characters wouldn’t normally interact like they’re dating but they are because they want to help a friend avoid embarrassment, they’ve been hired to play the role of ‘date’, or they’ve struck a bargain and get something out of it. In all three examples we’ve got characters doing out of character things for plot reasons but they have a deeper motivation. In the first instance, the audience might say ‘these characters are meant to be friends but they’re acting like romantic partners’ but they can also say ‘they’re doing this so Character A isn’t shamed by the other guests for being single’. They’re serving the plot but they’re also doing it for a reason that makes logical sense for the characters.
If these characters had no connection it would be difficult to make their ‘fake date’ believable because they wouldn’t have a reason to go through with it. Would they ask a random person in a shop to be their fake date for a wedding? Probably not, it’s creepy for a start. So why would they pick their fake date? Why would their fake date go along with the plan? We can even get away with ‘because they think it would be a laugh’ if we create a character for whom that would be a convincing motive.
Our characters and plot need a logic, it doesn’t have to be real world logic but it needs to follow the logic of the world of our story. If our characters don’t have logic (their own logic) then readers won’t be convinced by their actions and potentially frustrated by their lack of consistent characterisation.