A want is external, stated, and chaseable: win the case, get the money, reach the border. It gives your story a finish line and lets the audience keep score. A need is internal and usually invisible to the character: forgive the father, stop performing, admit the fear. It gives your story a reason to exist.
The engine is the gap between them. When the want and the need pull in opposite directions, every scene can force a small choice, and the accumulation of those choices is what an audience calls character. If your protagonist's want and need point the same way, there is nothing to dramatise — they simply succeed and the story flattens into a series of events.
The climax is where the two collide and the character must sacrifice one. Give up the want to get the need and you have written a redemption. Take the want and refuse the need and you have written a tragedy. Both work. What does not work is getting both handed over for free, which is the single most common cause of a hollow final ten pages.
Reactivity is almost always a want problem, not a personality problem. A character who does not want something specific enough to act cannot drive a story, so the plot starts pushing them around and readers feel it immediately, even if they cannot name it.