I got one of my best lessons in writing characters in fiction long ago in a basic theater class.
Actors are taught right off that to play a character, they must figure out what the character is trying to do. Stanislavski, the great acting guru, said that the best way to handle motivation was to phrase intention in one of these two ways: I wish to _______ or I want to ________ followed by an active verb.
Actors learn that character motivations shift from moment to moment (often should shift on every line), but that the motivations are all linked. So in one second, a character is thinking/playing, "I want to shame him into telling the truth." The next second is, "I want to show him I'm smarter than he thinks." And then, perhaps, "I want to leave him in the dust." These moment by moment beats might be fitted under a larger umbrella (or in Stanislavki's words, a "superobjective") like "I want to improve this relationship."
The trouble is that often new actors and new writers forget that the verbs are supposed to be active. The important thing for them to learn is to get rid of all BE verbs. I want to be happy is not very useful to the workings of story. Oh, sure, a character can hope to be happy in the future -- eventually. But a story is about conflict and pursuits and about those damned obstacles in the way of the goals. And a story doesn't operate very well on BE verbs. I want to be rich. I want to be happy. I want to be respected. Who doesn't?
The story's job is to show how a character tries to affect circumstances, to change the status quo, to manipulate other people toward the intended goal. Manipulate? Sounds so un-nice, doesn't it? But that's why obstacles are important and why the stakes have to be high. If an obstacle is good enough, we will think, "Yes, in that circumstance I behave just at that character does." We are more likely to be hooked when we watch someone doing various actions out of anxiety or desperation in order to get the money or happiness or respect.
Don't discount the moral aspect here. The stakes and the obstacles must be weighed against the desperation of the intentions/motivations. After all, Hamlet makes a mess of things in that first court scene. He makes a fool of himself, shames his uncle and mother. Actions that backfire or otherwise inch the plot forward toward more trouble are the good stuff of story.
And don't discount irony. When people do things to get what they want, it might very well turn out to be... well, the wrong thing.
But the word BE should signal a warning that a character's objectives are undefined, perhaps even static, and that the details of motivation still need to be worked out. I sometimes think that to intensify motives (far along in the story) the phrasing should be "I must" followed by an active verb. I must disentangle myself from the lies. I must end this friendship forever. I must defuse the bomb. These are likely the high point moments.
But even early on the active verbs are necessary. "I want to stop him from buying a new TV" and "I want to get extra hours of work to get us both of us out of debt" are better story fodder than "I want us to be happy and financially comfortable one day soon."
Another of Stanislavski's brilliant warnings is that an actor (thus, a character) should avoid wanting the other character to do something active. "I want him to get extra hours of work" is not active or useful. Instead, it should be "I want to persuade him (even though there are many obstacles) to take extra hours of work."
That's the difference. I love Stanislavski for this simple, profound lesson.