Einstein in a Football Tee

It is fairly safe to say that no author creates characters entirely from scratch. Every character is usually based on some aspect of someone. People watching in a public place is one way that. There is an endless stream of story fodder marching by and the author is for the most part unhampered by the actual facts of their lifer.
For instance; if an author wanted to say that that old man who walked by was Einstein, time traveling after faking his death and discovering the secret to it all, well that is just fine. He could even have developed a liking for a specific sports team to explain that shirt he is wearing.