Tolkien’s Lord of the Ring series is an excellent example. Frodo must go from a simple, happy Hobbit to a seasoned, bone-weary, and world-wise traveler. Of course he has three volumes to do it in, but grow he must, and grow he does.
There are stories where this is done in extreme, and the result is comedic. While I love the John Ritter and James Belushi film, “Real Men“, but the transformation from Milk Toast to He-Man Special Agent is too fast and unbelievable.
Unless you are writing about a Superman-type character, your MC will need to grow throughout the book. Only special proto-type characters can stay the same. In such a case the supporting characters need to change. Someone has to grow, has to change, during the story, or you find yourself slipping into Experimental Fiction.
The change has to be subtle, by the way, in order to be believable. Also, do yourself a favor, and don’t use the hackneyed old bit about guy meets girl and they hate each other only to fall in love. Find a different way, OK? I mean, really, what’s the first thing that comes to mind when boy meets girl and the sparks that fly are ugly? Yeah. That’s it. They’re gonna be an item.
People might fall in lust right away, but love takes longer than that. Let things perk. Let them develop.
The trick is to know what the change will look like once it is complete. Then, ask, will it occur half-way through the story? Three quarters? Will it take the whole book? Will the character have to die, and in dying see the light? If you know where the character is going, the chances are good that you can figure out how he or she will get there.