This was snapped during a lunch. Total shooting time—a few seconds.
We so often read that a proper "portrait" should be snapped with a longer than normal lens, a low ISO to get lots of detail, and have a soft light held up above the head, and slightly to the side.
The key, in my opinion, is always carry a camera. Have your camera available to capture candid, authentic photographs. Available light, no posing. This portrait used 2000 ISO, the lens wide open at f4, and 1/100 sec. to stop any movement. I didn't even take time to compose—I just snapped. I leave the "Face Detection" on unless I'm photographing a landscape or subject other than a person.
The GFX100RF has the equivalent of a 28mm lens. The large sensor renders fine detail even at fairly high ISO ratings. And the drawing of the lens is just perfect in my opinion. It was set to B & W, with slightly reduced sharpness and clarity (set in-camera). Ideal for "portraits." Now, for some subjects I will likely increase the sharpness and clarity to the normal setting. The camera is new, and I'm still experimenting with it.