Taking a good portrait can make an image look incredible.
Portrait lighting is a mixture of mood, convenience and demand. At times you will be handed down a very specific requirement, high school senior portraits, corporate headshots, engagement shots. There is literally no room for experimentation in such situations. In some situations you do have room for experimentation but that is very limited. Such as baby portraits, or engagement sessions. You can experiment with the light but you cannot go overboard with your trials. The only time you can really do that is when you are shooting for model portfolios or when shooting for your own project.
Beyond lighting a portrait photography session involves a lot more. The right lens for example, determines the look and feel of the image in question.
How exactly? The right lens will capture the image in the right perspective. Let's take an example to demonstrate this. Let's say that you are shooting a female model in-studio. Let's also assume that she has double chin. So, in this case you have to be mindful of the body shape as well as shape of the subject's face. Not only will you have to work on the pose of the subject, but you will also have to light up the subject's face in the right way so as not to exaggerate the double chin. You will also have to use the right lens. How exactly?
First thing is place the key light slightly higher up. That way a pronounced shadow will form just under the chin. This will obliterate the double chin. Next, change the pose. Instead of a straight-on look, you could ask the subject to stand facing camera right or left and then turn her face towards the camera. That will further obscure the double chin.
The other thing that you could do is shoot from further away using a mid-range telephoto lens like the 135mm.