Characters must always be in the driver’s seat of your story, your plot… you, the writer, must be a silent passenger.
A story is believable only if it appears to your audience that your characters are acting and reacting because of their inner motivations and the situations in which they find themselves.
When it becomes obvious that characters are doing what they do and saying what they say because the writer is manipulating them like marionettes, they’ve then lost their credibility, and so has the story.
Characters should not ever do things or say things because the writer needs them to do or say those things in order for the story or the plot to work.
Nor should characters do or say things because the writer is behind them pushing the writer’s own cause or agenda. The audience will immediately know on some level that the character is being controlled for the writer’s purposes.
And when that happens, audience disbelief (and annoyance) will come rushing in like a tsunami.
A writer has to create his/her characters in such a way that we, the audience, believe those characters are real live human beings with strengths and flaws, just like the we, the audience members are.
A writer has to imbue his/her characters with the characters’ own needs, wants, and desires based on the personalities with which they are created.
Because, when characters do or say things that stem from the characters’ own motivation, the characters’ own needs, wants or desires that’s what gives the illusion of reality to those made-up characters.
The classic: THE WIZARD OF OZ.
Dorothy is motivated to do and say the things she does, at first, out of her desire to see if there’s a better life “Somewhere over the Rainbow.” Then later, she’s motivated by her want to find her way back home to Kansas, and her desire to help her traveling companions.
The Tin Man acts and speaks from his want for a brain, and his need to protect Dorothy.
The Wicked Witch is motivated by her need to avenge her sister’s death, and the desire to possess the ruby slippers.
Of course everything — characters, their needs, wants and desires, their motivations, the situations in which they find themselves, the story, the plot, — always come from you, the mind of the writer.
But, it should never, ever appear that way to the audience