Character Arc

In dramatic writing, the very essence is character change. The character at the end is not the same as he was at the beginning. He’s changed-psychologically, maybe even physically.  - Robert Towne

Good movies are about an interesting and flawed somebody (your protagonist), who wants something badly (goal) and is having trouble getting it (obstacles). By the end of this journey, however, your main character or characters should be different because of the experience. If you don't' show the possibility of moral transformation or an increase in wisdom in your protagonist(s), there really is no point in writing the screenplay at all, because one of the most fundamental human principles is that human beings do have the capacity to change. This is the character arc.

But is there a difference between growth and change?

I say yes; it's not just semantics.

Take the 1957 drama Sweet Smell of Success. The protagonist and anti-hero Sidney Falco (Tony Curtis) does not change in the end. He's still the same guy: “a cookie full of arsenic” as the antagonist J.J. Hunsecker (Burt Lancaster) describes him; however, he does learn something. He doesn't respect J.J., but he's willing to "jump through burning hoops like a trained poodle" for J.J. in order to ensure his own professional gain, but in the end he learns he can't trust a man like J.J., who has "more twists than a barrel of pretzels." Knowledge and self-awareness is growth, and Sydney does grow within the small fishbowl of the "slimy trade" he operates in, but there is little hope that he'll live his life differently tomorrow.

Susan Hunsecker (Susan Harrison), on the other hand, the meek, intimidated, and terrified little sister to J.J. finally stands up for herself in the end, changing into a self-empowered, confident, strong woman. We are more than confident that tomorrow she will live differently. And this change was what we were hoping for the entire film.

So knowledge is growth, but acting upon the knowledge is change. You don't need both in your main characters, but you do need at least one.