The subject to background separation. This is one of the most important aspects of photography and one of the least talked about.
It’s so important because it can easily make or break the picture. You can have an awesome composition but if your subject does not stand out of the rest of the background, it kills the image.
Think of a black subject dressed in black and shot (photographed) against a dark background in the dark. A little over exaggeration but you get the idea.
I bring you one of my own pictures as an example.
The problem with this picture is that the face of this girl totally blends into the background. In fact, I can’t help that the white “Leitz” label gets more attention from me than it deserves.
Using a flash would’ve probably been enough to make the subject “pop”.
Compare that photo to this
Without flash, the subjects would blend in with the background.
Separating the subject from the background doesn’t necessarily mean you have to use flash. Of course not. Flash is just one way to do it.
E.g. this one below.
White background and dark subject. There is no blending into the background. Even though her face is not dark, it still pops out because the mirror on the photograph is basically pure white.
So look for either white subject-dark background or dark subject-white background situations.
If your friend is dressed in dark clothes, put him against the light background and vice versa.