Question: The story I am writing is to be a catharsis. Already the story (basically) has already been set. The main characters 4 of my principle characters have been in my head for years. They still to be developed. As the story moves forward, I place characters to support the situation I've decided to create. Mostly it will be a situation where I want the principle characters in a predicament that has the affect of making them unsure of themselves, the rock and the hard place scenario. Is it possible to write a book in this fashion? I have what I believe to be a good beginning. Now that I read the things said here, I've become concerned with the characters development. Is there a way to change the characters, (if really and truly that needs to happen) without changing the story.
I can't be quite sure from your description whether you have developed both an external plot and the characters' internal conflict (being unsure sounds internal).
There's nothing wrong with designing characters to fit the needs of the external plot. However, there are times when one realizes that a mismatch has developed.
For instance, in the process of letting your characters develop authentically, you may discover that they can no longer do what you originally envisioned them doing at the climax, so the external plot has to change. On the other hand, sometimes as you develop the external plot you discover that you need a different sort of main character to bring about the resolution you want, which may mean introducing another character earlier on, switching main (primary POV) characters, or changing your main character so he/she can be the type of person who will do what is required.
These are tough dilemmas, and there is no right or
wrong way to resolve them. However, bear in mind that, for most readers, what matters most is an authentic character. Readers can handle a slightly unrealistic plot if the characters feel authentic. But the most well thought out plot can fall flat if the characters feel inauthentic.
If you feel that the external plot matters more to you, you might consider the climax of the story as the lynchpin to which everything builds and which will guarantee the ending you want.
Map out the arc of the overall plot with the climax in mind. What has to happen at each step along the way to build to that climax?
Then do the same for the main character. What must that person do at the climax? To do what is required, will they have to change their approach, grow as a person, or give up something that is holding them back? Or will they need to grow in their resolve to not change, to stay true to themselves?
Map out that character's arc. Make sure the reader sees who they are in the beginning, how they are pressured to change, whether they change, and how they feel at the end of the story.
You may also want to map out the other characters's arcs, the impact character's arc, and the relationship arc.
On the other hand...
If you feel your main character's journey and who that person is to be the most important part of your story, then perhaps you should make his/her personal crisis (the moment when he decides whether or not to change) the lynchpin. In that case, you might alter your external plot and the other throughlines to serve the goal of bringing about and resolving the main character's crisis in the manner you think is important.
Best of luck.