If I want a grainy image- then I shoot to underexpose a little and bring it back up (yes I do this on purpose sometimes)- love me some gritty grain (or you can fake it and add grain in LR or PS later).
But back to the portrait type. This type of image you generally focus on eyes, you want catch-lights in the eyes if you can and nice bright mid-tones with even lighting on the face (as light allows). This is preference and will vary on lighting, subject, time of day to name just a few things. I generally shoot with apertures f/1.4-f/2.8 for single subjects. These apertures generally allow me nice smooth skin, with little skin detail and I shoot to overexpose a little bit on the skin and pull it back in Lightroom. If I want a grainy image- then I shoot to underexpose a little and bring it back up (yes I do this on purpose sometimes)- love me some gritty grain (or you can fake it and add grain in LR or PS later).