A is quietly browsing in a bookshop when they observe B take a book and hide it under their coat. Quickly list 10 - yes, 10! - possible things for A to do next, leaving a space between each one. Now write alongside each of the list why someone might behave like in that way, listing as many reasons as you can. For example, if you've written A tells the bookshop manager, then what sort of person reports a shoplifter? They could be ultra law abiding, or like stirring trouble, or bored with their lives and want to see some action.
You should have a piece of paper covered with actions, and possible reasons. Choose one reason that you like the look of. Write down how you think A looks - clothes, hair style, what they've got with them. Finally, write down what A was doing in the bookshop in the first place.
What you've done is work backwards through the usual process. We often start writing with a character and then have to think of a scenario for them to take part in. But if you think about it, in real life we learn about people's characters through their actions. So, if you start with your character taking action, you get to know them much more quickly and descriptions like their clothes etc fall into place more easily.
Enough of the explanations - write this scene and see where it leads you, and if you get bored, try giving A a different action.