Main Characters do not need to change in order to grow. They can grow in their resolve.
The Main Character represents the audience's position in the story. Therefore, whether he changes or not has a huge impact on the audience's story experience and the message you are sending to it.
Some Main Characters grow to the point of changing their nature or attitude regarding a central personal issue like Scrooge in A Christmas Carol. Others grow in their resolve, holding onto their nature or attitude against all obstacles like Dr. Richard Kimble in The Fugitive.
Change can be good if the character is on the wrong track to begin with. It can also be bad if the character was on the right track. Similarly, remaining Steadfast is good if the character is on the right track, but bad if he is misguided or mistaken.
Think about the message you want to send to your audience, and whether the Main Character's path should represent the proper or improper way of dealing with the story's central issue. Then select a changing or steadfast Main Character accordingly.
The Theory Behind It:
Do you want your story to bring your audience to a point of change or to reinforce its current view? Oddly enough, choosing a steadfast Main Character may bring an audience to change and choosing a change character may influence the audience to remain steadfast. Why? It depends upon whether or not your audience shares the Main Character's point of view to begin with.
Suppose your audience and your Main Character do NOT agree in attitudes about the central issue of the story. Even so, the audience will still identify with the Main Character because he represents the audience's position in the story. So, if the Main Character grows in resolve to remain steadfast and succeeds, then the message to your audience is, "Change and adopt the Main Character's view if you wish to succeed in similar situations."
Clearly, since either change or steadfast can lead to either success or failure in a story, when you factor in where the audience stands a great number of different kinds of audience impact can be created by your choice.
In answering this question, therefore, consider not only what you want your Main Character to do as an individual, but also how that influences your story's message and where your audience stands in regard to that issue to begin with.
Just because a Main Character ultimately remains steadfast does not mean he never considers changing. Similarly, a Change Main Character does not have to be changing all the time. In fact, that is the conflict with which he is constantly faced: to stick it out or to alter his approach in the face of ever-increasing opposition.
Illustrating your Main Character as wavering can make him much more human. Still, if his motivation is strong enough, your Main Character may hold the course or move toward change from the opening scene to the denouement. It all depends on the kind of experience you wish to create for your audience.
There is no right or wrong degree of certainty or stability in a Main Character. Just make it clear to your audience by the end of the story if he has been changed or not by the experience. Sometimes this happens by forcing your Main Character to make a choice between his old way of doing things or a new way. Another way of illustrating your Main Character's resolve is to establish his reaction in a particular kind of situation at the beginning of the story that tells us something about his nature. After the story's climax, you can bring back a similar kind of situation and see if he reacts the same way or not. From this, your audience will determine if he has Changed or remained Steadfast.
What if a Main Character Changes when he should Remain Steadfast, or Remains Steadfast when he should Change? Choosing your Main Character's Resolve describes what your Main Character does without placing a value judgment on him. The appropriateness of his Resolve is determined by other dynamics in your story which will be addressed later. For now, simply choose if your Main Character's nature has Changed or Remained Steadfast.
Practical Examples of Change Characters (provided by our interns):
At the end of the story, the Main Character's basic way of seeing things has changed from what it was at the beginning of the story. For example, a stubborn bounty hunter, who sees every criminal as "guilty," changes to realize this isn't true for every criminal and decides that he is chasing an innocent man; a woman who has always put her job before her family changes, and puts her family first by adapting her schedule so she can spend more time with her husband, even though it will mean missing a promotion; etc.
Practical Examples of Steadfast Characters (provided by our interns):
At the end of the story, the Main Character's basic way of seeing things has remained the same as it was at the beginning of the story. For example, a man wrongly accused of murdering his wife remains steadfast in his pursuit of the real killer believing this will eventually solve his problems; Despite all attempts to convert her, a woman remains true to her faith in her religion believing her god will protect her; etc.