I went into The Decalogue with the impression that many characters reappeared in more than one episode, and imagined a spectacular and intricate interweaving of lives and stories. Unfortunately, not so: that wonderful series of films remains to be made. I worked my way through the cast lists of the various episodes of this wonderful series of films and turned up the following multiple appearances;
Multiple AppearancesArtur Barcis (1 Man in the sheepskin, 3 Tram Driver; 2,4,5,6,8,9 as "Young Man")
Krystyna Janda (2 Dorota, 5 Taxi Woman)
Olgierd Lukaszewicz (2 Andrzej, 5 Taxi man)
Stanislaw Gawlik (2 unnamed, 6 Postman)
Krzysztof Kumor 2, 3
Joanna Szcxepkowska (3 Janusz's wife, Unnamed 8)
Jan Tesarz (4 Taxi driver, 5 Taxi Driver)
Olaf Lubaszenko (6 Tomek, 10 Tomek)
Piotr Machalica (6 Roman, 9 Roman)
Katarzyna Piwowarczyk (7 Ania, 9 Ania)
So even apart from "The Watcher" (Artur Barcis), every episode includes at least one character from another episode, but nobody appears in more than two apart from "The Watcher." As far as "The Watcher" is concerned, here's what Joseph G. Kickasola writes in a footnote to the article posted at metaphilm;
QUOTE
Kieslowski experienced technical difficulties including him in Decalogue VII, and chose not to show him amid the dark-humor atmosphere of X, a decision he later regretted (in Stok, Kieslowski on Kieslowski, 158)