There's one essential rule for plot: by the end of the book, the main character must undergo a recognizable change.
However, this change has to flow naturally in a way that's true to the protagonist's nature. Conflict is another essential plot element, and the events of the story should push the character into difficult situations that act as a catalyst for change. However, the character's reaction to difficulty shouldn't represent a dramatic departure from weakness to sudden strength. If she's used to crawling, for example, she shouldn't suddenly fly.
For example, if a shy outsider is forced to speak in front of a school auditorium full of his peers, this can't be an easy experience. If his speech has the polish of a presidential address. Otherwise, there's no conflict or believability.
As an article in the January 2013 Writer's Digest puts it, "Characters who demonstrate instant skill or comfort with something they've never tried before resides largely in the realm of shlock. The less familiar the behavior, the clumsier and more uncomfortable it should be."*
That same awkward teen should have an experience similar to a moutain climber's difficult ascent. As he stumbles and stutters through his speech, the scene tension instantly increases, as does reader empathy.* Then, when he succeeds in rallying the student body, his victory will feel earned rather than forced.
"Push Your Characters to Their Limits," David Corbett, Writers Digest, January 2013, p. 32.