Your protagonist, indeed all your characters, at any moment in the story should take the easiest route, from his/her point of view, in the pursuit of their desires. It is human nature. But of course, what is the easiest route is relative to each character.
What generally happens in life is that we take an action to achieve our desire or goal, while thinking: If I do this conservative action, the world will react in such a way that I will move a step closer to achieving my goal. And in life, if we’ve thought it out, we are usually right. This is how we want life to work, we think the problem through, we take action, and we get the desired results. But we NEVER want that to happen in our stories.
In your story, you need to concentrate on the moments where the character takes an action, expecting a helpful reaction from his/her environment, but instead the action provokes an unexpected, more powerful response. That produces conflict, and conflict is what makes the story interesting.
For example: I see a younger man in a bar. He looks my way, smiles. I think to myself, Cool, he likes older men. My desire is to take him to bed. I know my first action should be to strike up a conversation, so I walk over, smile and say, “Haven’t we met before?” – and he shouts, “Don’t you wish, grandpa.” Then turns his back to me. Suddenly, the scene is more interesting, because in order to get what I want, I have to do something more forceful than my first minimum action.
When that happens, when the protagonist takes actions that s/he thinks will move him/her closer to an object of desire beyond their reach, and gets an unexpected reaction that pushes him/her further away from said desire, it creates a gap between the character’s subjective thought and their objective reality. This gap is where the story should focus. In fact, this gap IS the story.
So what happens when a character finds him/herself caught in this gap? Simple, s/he has to regroup. The world is now different from before the character took the first action. The character must assimilate the change, then decide on a bigger, bolder plan of action to achieve the goal. Then they must take action again. But, of course, the same thing must happen. The world must react in an unexpected way to this new action, pushing the character even further away from the prize. Thus, the gap widens further, creating the need for even more dramatic action.
In our example: assuming I still want to bed the young man who insulted me at the bar, I might lay a hundred-dollar bill on the bar and say, “Let me buy you a drink.” And after the bartender brings a round of drinks, I tell him, “Keep the change.” So the young man knocks back the drink, then turns to me as says, “I’m no whore, old man. I can’t be bought.” Now the stakes are raised, I’ve gambled $100 without getting what I want, and what will I do next? At this point the reader should assume I’m wasting my time and money.
Two key things happen when the character takes this second action. 1) the stakes are raised and the tension level goes way up. He is doing much more than he originally wanted to do, but he is now committing himself. And 2) by committing himself, he opens him self up to risk. This is not only key, it is a pillar of good fiction. The second action MUST put the character in a position where it forces him to dig much more deeply into his human capacity, or stand to lose something valuable in order to gain what he covets. In short, the protagonist, now in a state of jeopardy, must risk something he already has, in order to gain the thing he desires.
The measure of the value of a character’s desire is in direct proportion to the risk s/he is willing to take to achieve it. The greater the value, the greater the risk.
So again, the protagonist must take a much more dramatic, risky action. And, of course, YOU NEVER GIVE THE PROTAGONIST WHAT HE THINKS HE WILL GET. You always want to keep that gap getting even wider with each action. Every time s/he takes action, the gap should widen, pushing them further from the goal, until the end when subjective and object collide head on. That is what keeps the tension in a story escalating. At some point, it should seem like the protagonist will surely lose what s/he has risked. But of course, they may or may not, depending on the story.
So, I know what you're thinking: what was my next step at the bar, and did I succeed in getting that young man in my bed. If you are thinking that, it's because of good story telling. You want to know what happens next...