@InfraBlue,
Ok, I can see if I gaze at the lone man for a moment how he could be sinister. In that I could get the impression he could be following "casing" the other 2. Even that doesn't ring true to me though, because since there are so few people up and about, and in the diner, it would be pretty obvious if he was following them.
My initial take on the man, at a glance, was that he had just stopped for a cup of coffee, on his way to somewhere else.
One thing that sticks out to me, considering the dress (indicating around what year this picture is set in), the time of day/night, the location, is that no one is smoking.
I think the man an the woman are very interesting. I think there's a comfortable romantic relationship between the 2. Look how her arm is bent under the other, to reach out to him. If she was just sitting with her arms folded, the arm wouldn't be moved over that far. He's not necessarily reaching back out to her, but that doesn't mean he isn't fond of her. They are both just thinking their own thoughts.
That's one of the big appeals for me. Everyone is thinking, not feeling the need to interact, they are just taking the opportunity to "be"
I'm betting the counterman knows all these people, like they stop by every couple weeks or so. The customers seem very familiar, at home there.
What time of the night do you think this picture is in?
I'm thinking 1:34 a.m.