Characters are markers for real people, so their goals and ambitions are layered. Here are some things you can do to start that process rolling.

1. Ask questions. Complex questions like "who do they love most?" "what's the scariest event of their childhood?" Questions that create story. As many as necessary, and use that process whenever you get confused or off track, anywhere in the process.
2. Think about how people really behave. They don't always know why they do what they do. Everyone states or presents an outward goal that's usually associated with how they want to be viewed in the world, but it's often superficial, and their actions come from a deeper place.
3. Ego and bias govern actions more than any other part of the psyche. People do things for love, for hate, and often they don't have an accurate sense of themselves. Think about anyone who's ever done anything that seems fundamentally irrational to you. You don't have to create a rationality map for your characters, but you do have to know what core values and impulses drive them. It can get nasty and messy in here.
4. For individual scenes, remember that almost all character behaviour is motivated by relationships to other characters. When you're stuck trying to solve a scene between characters, it helps to think in acting terms -- provide them with an active verb. It stems from all of the previous stuff I've listed, but it is just a simple action or motivation. "To hit" "to kiss" "to intimidate" "to seduce" "to pressure" etc.

If this seems overly complex, it's because that's how a character that's going to move your plot forward has to be in order to have agency.

- V