From the start, let's assume we have a portrait that has been correctly focused, and somewhat well exposed...not too light, not too dark. What else, above these two basics make for a portrait worthy of looking at more than once? A lot is subject to personal preference, but here are some of my thoughts on the matter.
--Composition. One of the first things a viewer will notice about a portrait, whether consciously or subconsciously, is how the subject is placed within the borders of the image. Is the head of the subject high or low in the frame? Are the eyes located in the middle of the frame? How much breathing room does the subject have? These are elements that play on the mind's eye, usually unwittingly, and contribute to how favorably a portrait is perceived overall. My thoughts: The eyes of the subject in a head and shoulders portrait should be located in the top third (approximately) of the frame. You will notice a slight visual discomfort (if you pay attention very closely) when you view a portrait where the eyes are located half way down the image. You may feel like lifting the subject up into the upper third. If you feel this, great! The ancient Greek artists were big on placing important areas of focus at one of 4 points of intersection that are created when you divide a frame into 9 squares. You may know this as the "rule of thirds".
I like to place the eyes somewhere near the two upper points of intersection in a head and shoulders portrait. Of course, there are times when going against this "rule" will work. Knowing when it works and when it doesn't will come with experience.
--Appropriate lighting. I don't mean lighting that
one "should" achieve for all portraits, but instead,
lighting that compliments the specific portrait we're attempting
to create. For example, a portrait aimed at displaying how soft
and tender a subject is should, perhaps, use soft lighting and
have a delicate feel to it. Whereas, a subject with a harder
feel, or more sharp features, should have more dramatic and hard
lighting...perhaps more shadows. In order to decide which treatment
to give a portrait, it's good to first examine how you feel about
the subject. What do you want to convey? By observing your own
feelings on a subject, you will be more able to decide: hard
or soft, shadows or even lighting, rough or smooth, etc.
--Non-competing background. This is a toughie. How should you render the background? Should it be in focus, blurred, dark, light? My view: It should not be distracting. It should provide a good landing area for the subject. Personally, I prefer soft, blurred backgrounds in outdoor portraits. I do this by using a wide lens aperture combined with a long focal length. By placing the subject somewhat close to me, but far from the background, a blurred background is achieved due to a shallow depth of field. A blurred background does a nice job of setting off the subject in the foreground.
For indoors in studio situations, I prefer either a light or dark treatment to the background, rarely anything in between. Keeping my choices limited to about two backgrounds, more attention can be given to capturing the personalities of the subjects...catching a special moment, instead of fiddling around with background issues.
--Timeless/Classic clothing. This is very much a personal preference thing. I treat the clothing a subject wears just like I do the background: I want it simple, non distracting. This way the image will more likely be good for all time, and minimize the chances that the image will be "dated" due to trendy clothing. I don't like to tell a client what to wear for photo sessions, but I do tell them what I'd prefer.
--Captivating expression. To me, good portraiture is
all about expression. The technical part of a portrait could
be perfectly executed, but if the expression on the subject is
contrived, unnatural, or not believable, then the portrait won't
"work". One thing I always do while shooting my portraits
is to trip the shutter at the instant the subject forgets or
neglects to pose for me. In other words, I shoot in between the
poses. With children, I sometimes tell them to wait a second
while I focus...and then, while they're waiting and before they
know any better, I click the shutter.
--Nuance in posture. This is closely related to capturing a good expression. Getting the subject to assume a natural posture takes the same care that getting a natural expression does. I use the same technique for this as for expressions. I shoot in between the stiff poses, and try to grab the moment when the subjects let down their guard...when their bodies assume a graceful state of repose. The image below illustrates subtle postures, but it took some doing to get them to settle in so that the final image would look relaxed.
--Catchlights in the eyes. I think catchlights in the eyes are very important in portraiture. Catchlights are those bright reflections you see in the eyes of the subject...sometimes they are square if the light source was a window or square light box, or they can be round if the light source was an umbrella light. The closer the light source is to the subject, the larger the catchlights...I like big catchlights myself, so I try to place the light source very close to the subject. Since I prefer square catchlights (because they mimic what would occur if the subject were near a window), I use a square softbox light setup (see image below). To me, a large round catchlight indoors is unnatural...since it would not usually occur in the average room. Next time you find yourself indoors with a friend, look at his/her eyes...what shape are the catchlights? I like to observe the catchlights in the eyes of people I meet, wherever they happen to be...it makes for good practice in "seeing" photographically.
Photo courtesy of www.photoflex.com
Final thought: When I set up to create a portrait for a client, I try to forget all of the portraits I've seen in the past...those of others, and also those that I myself have made in the past. I want to create something new, something that doesn't just fit nicely into what a portrait should be. Proceeding in this way leaves a photographer open to creating a portrait that the viewer is more likely to find interesting...and worthy of viewing more than once.