If you want to make your story interesting, your characters are going to need some kind of conflict or crisis. There are stories where characters have really subtle character arcs. There are also stories where the characters more or less remains static but their multiple facets are explored in a lot of different ways throughout the story. However, you still need to have conflict in a story. If it’s told well enough, your characters will react to that conflict and probably change as a result to a certain extent.
The character development struggle that you create is more or less the entire story in most cases. This is the heart and the meat of the story. As such, you really have to make sure that you choose the right character development struggle.
Having characters that barely change can be a problem, especially if these are very flat characters in general. Rounded characters can stay the same while people explore their many facets. Flat characters can become deeper with character development. Flat characters that don’t develop won’t star in interesting stories, no matter how good the actual prose or dialogue might be. You need solid characterization and character development in order to make a story work.
However, characters that change too much can also be problematic. You want the change to be believable. If your character seems like a completely different person at the end of the book, you’ve gone too far. Of course, there are situations in which people will change dramatically in real life, so extreme character development actually can be done well. Most things in fiction can be done well in the case of the people who are trying to find a way to make a difficult plot line work. However, it’s still important for people to keep in mind some of the central tenets of what makes a good story.
Striking the balance between subtle and believable character development and character development that people can actually see is going to be difficult at the best of times. There will certainly always be people who swear that even the most subtle and effective character development is actually heavy-handed. There are people who will say that heavy-handed character development is still somehow effective. As such, opinions about this sort of this are going to vary tremendously.
As such, it’s a good idea for people to keep in mind that all they can do is to try to stick to some basic guidelines about character development. They’re never going to please everyone and they’re never going to get to the point where they can make their writing immune to criticism. They can only try to get to the point where they can feel like they have succeeded according to their own standards and that they have still created an entertaining story.