I think you ought to read at least five or six of Kirk Tuck's blog articles on portraiture. He has some superbly-related insight, based on something like 30 years of commercial portraiture. Here are some search results from The Visual Science Lab, the name of his blog, as it relates to portraiture.
Visual Science Lab on portraits - Google Search
Not too many days ago, he published a sort of "here's how a portrait session really goes," blog entry, and it's a lot like my experience: people at first pose for the camera; the do all the poses they've done their whole life, and then, after maybe an hour of actual session time, there's a sea change, and their real, inner self reveals itself in a fantastic shot or two, and then...the remaining images are lesser, of lesser importance, of lesser quality, and it's time to end the session.
Not sure if there are ANY rules except, "have film in a film camera, and memory in a digital camera," and after that, well...a portrait session can be conducted in myriad ways, for times ranging from one minute to three hours, depending on multiple factors. I think Tuck is correct though, in that the best images usually do not happen immediately with real people (meaning people who are NOT actors, nor celebs, nor professional
posers).
For me, the biggest thing is
building rapport, and that begins with explaining HOW I will conduct the shoot, meaning how fast we will fire the frames, how the poses will be determined and directed, and how expressions should most definitely NOT always be a big, herkin' smile. My experience is that many older people who grew up in the film era are often, at some level, uptight about picture-making, and they fret about blinks, weird looks, and so on. I take the first five minutes before shooting to go through exactly what the process will be like. After a few minutes, people begin to get into the rhythm. It's like a dance: there are at least two people, interacting with one another, in a sort of rhythmic, repeating social interaction. A dance.
It really,really, really helps a lot to show some excitement, some enthusiasm, as you shoot. Periodically say aloud, "Oh, now THAT's a good one! You're doing great!" That is actually an old selling method I was taught years ago--
building enthusiasm for the finished photographs, during the shoot. But it also builds confidence in the sitter's own mind. Today, we can literally SHOW the sitter a few frames on the rear LCD, and I think after the first 10 minutes, or even less, it's worth stopping and showing them the early take on the camera's LCD, and allowing that to build their faith in you, the lighting, their posing, and other similar things, as well as to allow them to literally see what their "look" is giving to the camera. Then, go back to shooting, but do not stop again until mid-shoot.
At mid-shoot, or at first wardrobe change,quickly run thru the take and pause at a few frames, and zoom in quickly and show them how awesome they look. After that, don't review any more, don't show, just finish the shoot, and show some enthusiasm occasionally.