Needs are under the surface. They are often hard to distinguish for ourselves. The same is true for your characters. Protagonists don’t start off the story knowing what they want and how to get it. Instead, they consciously go after their wants. And that ordeal usually satisfies their needs, though it doesn’t have to.
It’s easy to distinguish what the character wants. They are the goals characters actively pursue. In Jurassic Park, the characters want to survive. The sisters in Pride and Prejudice want to find husbands (or love in Elizabeth’s case) so that they are not destitute when their father dies. In Se7en, Somerset and Mills want to solve the crimes.
The needs are trickier because they are often hidden beneath the character’s sense of himself and the world. Mills in Se7en needs to see the world as it is. And, by the end, he is disillusioned of his sense of justice. His need, though negative, takes him to the brink of humanity, to the depths of human experience.
An internal desire, one they don’t often admit to themself, asks what it would take to “use the character up”, to completely satisfy their life.
In Mills’s case, he starts out as an idealistic young man filled with potential. His wife is pregnant, they have moved to a new city filled with opportunity, and he hopes to exact justice. By the end, his idealistic view of the world has come crashing down. He’s lost his wife, the child he didn’t know about, and his sense of right and wrong. He’s faced the devil himself and come out having lost.
Some characters don’t have an internal need. They might only work to solve crimes or navigate their experiences without fundamentally changing who they are. That doesn’t mean the story doesn’t affect them or that you shouldn’t consider who your character is under the surface. You still should. Mark Watney in The Martian doesn’t change who he is from start to finish. But, it’s because of who he is as a person that he’s able to survive the harsh environment of Mars.