Choose one of your fellow diners and write down: describe in detail what they look like, what they’re eating, how they eat, mannerisms, how they’re dressed, who they’re with, their idiosyncrasies that make them unique (do they frown, waggle their eyebrows, pop their knuckles, read, do crosswords, people watch, etc.), how they walk or sit, what makes them noticeable.
The reason I suggest a restaurant as opposed to a bus stop, train station or airport, is that they’re likely to be sitting there a bit longer, so you’ll have more time to observe them. It goes without saying that you should be discreet and not outright stare at them or you might be accused of being a stalker!
This is an easy way to build a believable incidental character for your story without having to imagine every detail about them.