In the fourth of his essays on Sukkot, “Seven Shepherds and Eight Princes of Men” (Change and Renewal: The Essence of the Jewish Holidays, Festivals and Days of Remembrance, trans. Daniel Haberman, ed. Yehudit Shabta, Shefa/Maggid, Jerusalem, 2011, pp. 117-22), R. Adin Steinsaltz considers the mystical custom of welcoming, in addition to real guests, metaphysical figures to one’s Sukka.
The custom to greet and acknowledge these “visitors” is recorded in works describing various Jewish observances, all citing this practice’s original source as the Zohar, e.g.:
Yalkut Yosef, Kitzur Shulchan Aruch, Orech Chayim, Laws of Sitting in the Sukka and Other Days of Sukkot, #11.
It is made clear in the holy Zohar, Parashat Emor, 103b, that the glorious holiness of the seven exalted, holy guests, i.e., “fathers” of the world (Avraham, Yitzchak, and Yaakov, along with), Moshe, Aharon, Yosef, and David, are present in the Sukka. Therefore, one who dwells in the Sukka, should do so in a dignified manner, with honor and glory, especially when the Sukka is large and its holiness therefore that much greater. This in accordance with the Talmud’s statement (Sukka 9a): The Name of Heaven Is Manifest on the constituent parts of the Sukka (hence the larger the Sukka, the more elements with which holiness will be associated). Especially, one should be careful to distance himself from the attribute of anger and holding everyone closely accountable to high standards within the Sukka, and one should therefore not raise his voice towards the members of his family. All of this is in honor of these “guests” of exalted esteem. (Shnai Luchot HaBrit, Bikurei Yaakov, #2.)
(A contemporary film that constitutes a “riff” on the custom is Shuli Rand’s “Ushpizin” [https://en.wikipedia.org/wiki/Ushpizin] to which reference in passing was made in https://yaakovbieler.wordpress.com/2017/10/04/agnons-the-etrog/ )