There are times when the lighting will remain relatively consistent for long stretches, or when the pose/shot is consistent and has an even-toned background that does not cause the meter to go up and down as the scene is photographed in different ways, such as zoomed in tight, or zoomed back to a wider point of view, and so on. If there will be NO zooming, you can easily use A or S or M or P exposure modes and arrive at the same,identical exposure with the simple click-click-click of a wheel on advanced cameras, or on the consumer or "one-button" models like the Canon Rebels and the Baby Nikons, you'll need to press a small button and simultaneously rotate the command wheel to get the exposure parameter(s) that you ACTUALLY want to have.
One thing I think about portraiture is that it's worth actually saying to the subject, "I'm going to take a close-up reading and evaluate the exposure. I'm going to come in close to your face and meter to get the EXACT exposure, then verify that, and then we'll start shooting for real." I actually say the above phrase. It explains what you are doing and why you're close to them with that 70-200 lens! And it also conveys that you are paying attention to technicals, and are meticulous.
Now, SOME cameras are very good at Aperture priority automatic, and do well against backlighting and can be relied upon to get the metering pretty close to right...other cameras, and other situations, will be clearly,clearly wrong. Again, how to meter depends on the lighting and conditions. Is it overcast? Metering is not that tricky. Strong, May through August backlighting down by the river? Exposure can be critical shooting against the light!!!
One big plus to shooting in Manual exposure mode is that you can get the right exposure dialed in, and then shoot, and the Lightroom processing will be identical for all frames in each exposure sequence/posing sequence, so you can optimize to the best frame in a sequence, and then copy the settings, select the other frames and then paste the setting to the similar frames. With today's better cameras, it's enough to have a consistent exposure, and then in software, equalize the shots with the right combination of software adjustments.