You can formally pose subjects or capture them candidly. You can choose to render them in Black and white or colour, include or exclude parts of them, locate them in a neutral of familiar surroundings, manipulate lighting in a studio or use natural lighting.
There are no rules, but each decision we make changes how the viewer perceives the photograph...and the person.
Fundamentally I believe a portrait should have the power to speak to you without words. It should say something about the person's character or their circumstance. Portraits are never just a visual record of a person.
A good portrait has the power to subconsciously convey something elemental of the sitter; their power, importance, virtue, beauty, wealth, taste, learning, trust.
It’s not a science, it's an art.