One of the harder bits of writing fiction and nonfiction is coming up with the individual personal details that create the cast of your book. This can be idiosyncrasies, unique facial expressions, time-period clothes, body type, etc. It is entirely possible to fall into cliches when putting together the characters that will inhabit your world.
There are resources to help, such as the series of master lists for writers, but one that I've found helpful is photography books and individual photographs. When I go to used bookstores I spend at least a little time in the photography section looking for collections such as those published by Phaidon and others where the primary focus is on candid shots of people who are not posed or in a studio.
Another resource is old photographs that can be found in secondhand stores, yard sales, and so on. These again often show people in their natural state or capture some moment of truth and expression.
The point is not to steal the person, but see within each face, body, posture, expression, etc. an opportunity to capture a unique personal trait that can be used to create true characters.