I'll address myself to one aspect of character development - the necessity to avoid putting the character in all the same (or very similar) scenes, acting and reacting in the same way every time he appears in the narrative.  We get an initial impression of him in his first scene and learn nothing new about him thereafter.  He always does and says XYZ in all of his scenes.  This is not "character consistency," it is "character paralysis."

Even minor characters who appear in few scenes do not need to be flat, one-facet characters.  If Bill Jones is seen in only three scenes, you can focus on a different aspect of his personality in each scene, thereby avoiding boredom and enriching Bill's character and your story overall.

An instructive example is the character of Ernie Macmillan, who appears periodically in the seven canon books, but as a flat, one-note character.  He is pompous (but not unkind; he doesn't put other people down), he brags about how many hours he studies for his O.W.L. exams, speaks of how important it is to plan for one's future, at the DA recruitment meeting he makes a speech about how important it is and wants people to acknowledge his speech by saying "Hear, hear!"  He frequently  refers to being a prefect (in case anyone had overlooked or forgotten that fact).

But what other facet is there of his character?  Stuff goes on during the first six canon years.  Do we see his reaction (or action) to any of those events?  No, he remains a minor character, and a boring one at that., a one-note character.  Our reaction to him, if we have any,  is lack of interest in him and dismissal of his role in the story.

In the overall story, Ernie is a caricature, not a developed character.  But in the end, after many hundreds of pages, he redeems himself in the Great Hall when the students are ordered to evacuate and he shouts, "And what if we want to stay and fight?"  Wait, who said that?  Am I hearing correctly?  Was that really Ernie Macmillan, the pompous, self-important, grade-grubbing kid I've known for so long?  Suddenly he's willing to risk throwing away his future, and even possibly his life, for a cause greater than himself?

Yes, Ernie's character can grow and develop if we, the authors, want it to, and we don't have to wait for the final scenes of the story for this to happen.

How can we achieve that?  One way is to put your character in a wide variety of situations and circumstances, not just (for Ernie) his classrooms and common room, or, for a romantic couple, not just endless scenes of angsty dialogue in the corridors of Hogwarts.  Send Ernie boating on the lake with a boatload of other students and then capsize the boat in the middle of the lake.  (It could happen, even to Ernie.)  What does he do and say?  He encounters a suicidal student who wants to throw herself off the Astronomy Tower.  Hew does he handle this?  Is he impelled to take follow-up action?  In The Order of the Phoenix Ernie joins the DA and presumably attends their practice sessions in the Room of Requirement, but the author never mentions his presence there  nor how he performs.  Well?  Badly? A quick study?  Frustrated by his failures? Helpful and sympathetic to others? Resolute?  Tempted to give up?  Jealous of others who do better and earn more praise?  In the overall canon, these scenes with the DA are a lost opportunity to expand Ernie's character and make him seem like a genuine person.