First: Protect yourself. There’s a fine line between being a Good Samaritan and needlessly putting yourself at risk. You won’t be able to help the other person if you are in danger as well, so take yourself out of harm’s way before calling 911 for help.

Then: Spend a few seconds memorizing a clue, like a physical characteristic of the suspect or a license-plate number, says Elaine Cloyd, president of Crime Stoppers USA. “In a time of distress,” she says, “it’s difficult to remember too much. But you might remember at least one really specific detail that can turn out to be important.”