This video “Portrait Painting From Life by Alex Tzavaras” demonstrates one method artists use to produce portraits, particularly in oils. It is called Alla Prima where the painting is completed in one sitting.
Oils are typically used due to the time that they remain workable as the technique relies on mixing the colours while they are wet. One of the distinct advantages of Alla Prima with oils is that it removes the risk of the paint cracking, as it is all dries together rather than in layers.
The technique demonstrated builds by “blocking in” the tones first (darks and lights) and then refining these with the mid tones. Notice that there is only a rough sketching of the outline using paint, rather than sketching the portrait in detail on the canvas prior to painting.