Frank Auerbach works over and over on a painting by scraping off the paint and reapplying thick paint. His drawings are built up of layers of paper, charcoal and chalk. In this way the surfaces and the palette change over time. In the artist’s own words: “Painting is not manufactured. Each picture haw it;s own history. The only contents are the fact that it always takes a long time, sometimes a very long time…”(https://www.tate.org.uk/art/artists/frank-auerbach-676/how-its-made-frank-auerbach)
Auerbach’s works and reworks repeatedly his drawings and paintings over a long period of time. His drawings evolve from repeated revision. (https://www.npg.org.uk/collections/search/person/mp06347/frank-auerbach)
His models remained the same for all the years of producing portraits. He believes that familiarity results in a more beautiful and instinctive image. Each painting is completed in a single session so that he retains the same state of mind aiming for cohesion and interdependence. To my understanding when a subject is familiar the artist creates an archive of images of that person as an archive in his mind and also through painting the same person over and over again. respectively each painting is built up of layers of all those ”snapshots” that are internalized to the artist regarding his model. This repetition of portraits of the same model create a narrative through the passage of time. (https://www.artsy.net/artwork/frank-auerbach-self-portrait, https://www.artsy.net/artwork/frank-auerbach-head-of-eow)

Self-portrait, 1958 Charcoal and chalk on paper
In the drawings like Head of E.O.W. 1959, 1959 and Self portrait, 1958,1958, layers of charcoal, patches of paper, worn paper and water-borne paper create a solid picture with great depth, rich texture and interesting mark making . Through the procedure of creating the drawing which takes actual time to be completed, the signs of time are part of the image and are additive to the essence of the depicted person.
Tim Adams write in The Observer regarding the work of Auerbach : “All of them dramatise the act of understanding the strange world beyond his eyes – you can look long and hard, for example, at his pair of “reclining heads” of his wife Julia, from the mid-1990s, stepping forward and back from the wall, before the expressive energies of his broad, up-close brushstrokes resolve themselves into something fleshy and living.” ( https://www.theguardian.com/artanddesign/2015/oct/11/frank-auerbach-tate-britain-review-60-years-retrospective). This is relevant to my thinking that having to look closer and deeper in the artwork/painting engages the viewer in a time related response. The time needed to look and observe the drawing or painting reflects the long time that took the artist to complete it. It is like the viewer is trying to follow the brushstrokes, the sacraping or any other act of the artist while making the artwork and tries to undercover the hidden layers under the thick paint or the layers of charcoal, watercolour or chalk on a drawing.
Regarding the difference between drawing from a photograph and drawing from life I would comment that the time spent is experienced in completely different way. Time spent in front of a person painting him or her carries a different energy that I believe is inevitably transferred to the portrait. The time spent demands devotion from both parts, the artist and the poser. A life portrait drawing is a dyadic action, while in drawing from photograph only the artist is present. In a life portrait drawing the passage of time is a shared experience that possibly is conveyed in the drawing or painting.