Think of three very different paths your protagonist might take. Project each one forward one or two big steps.
For instance, let's see I've set up a story opening in which Ian, my protagonist, is a widower nearing retirement age. He hasn't managed to save much money and his pension plan just collapsed. I have a sympathetic character and a situation to which (unfortunately) a lot of people can relate. But where to go with the story?
Scenario One: When he's approached by a younger friend who says there's a way Ian could earn $50,000 for one week of work, Ian knows it has to be something fishy. But he needs the money and decides it would be foolish not to at least hear what it's all about.
Scenario Two: Looking for an inexpensive way to have an evening out leads him to a senior citizens' social club where he turns out to be one of the very few men among a lot of widows. One quickly falls for him. She's rich and nice and would love to marry him. It would mean a comfortable retirement. He likes her but he doesn't really love her…does that still matter?
Scenario Three: Desperate for money, Ian signs up to be a subject of medical trials. When one of them goes terribly wrong due to negligence by the pharmaceuticals company, he has only one year to live. They offer him ten million dollars to shut up. He can have a dream life and money to leave to the grandchildren of his struggling son and daughter in law, but it will mean being complicit in the coverup of the company's practices which may go on to hurt others.
You can use the same strategy further along in the story. Let's say that you decided that Ian should go ahead and marry the widow--after all he does like her and has no intention of cheating her; he will be a good companion. But then…then we can think of three more options for what happens next:
1. She dies and he realises the rich thing was all an act. She had nothing but debts for which he is now responsible.
2. He meets another woman, and this time it IS love. But now he's spoken for…
3. He does marry her but the more he begins to actually love her the more suspicious she gets that he was after her money.
If you pay attention you'll get the sense that one of the options you generate is more right for you or your understanding of the character than the others.
(There are some writers who have tips for you on how to create great characters and plots. Who are they? Well, they include Dostoyevsky, Dumas, and Dickens. Also Calvino, Carver, and Cather. Their advice and how to apply it to your project are in my newest book, Your Creative Writing Masterclass. It's published by Nicholas Brealey and available from Amazon or your other favorite bookseller.)