Now figure out why they caught your eye. Was it their clothes? Hairstyle? The way they acted or walked?
Now, take one minute, in your mind, to conjure up their backstory, based just on what you saw. It can be as complete as from their childhood on, or just how their day has progressed up until you saw them.
Put yourself in that person's place...and have them look at YOU. How would they write YOUR backstory, that of a perfect stranger, in that one minute?
Have fun with this and don't overthink it. Some writers have a tendency to fill out these horribly complex character sheets with everything including favorite color, etc. But the characters are...blah. No life to them. A character is not just a set of facts and figures you can pull up out of a spreadsheet. They are their reactions to certain situations, their emotions. They are shopping trips to the mall and trying to make it through TSA without a full body cavity search.
When you're creating characters, sometimes we overthink them. Use short exercises like this to "blast" a character into creation instead of trying to spend years concocting them like some role-playing character sheet (roll D-12 for patience...).