Most of the characters are given specific flaws to help viewers distinguish them not only in their own worlds but also across timelines. The Unknown, the son of Jonas and Martha, has a cleft lip (sometimes also called a harelip).
The casting process was extremely meticulous, because the actors playing the same character in different time periods had to match exactly in facial structure and ear shape.
Most of the 'young' and 'old' actors are not related in real life; the only exception is the real-life father and son who play the trio of 'The Unknown' in Season 3.
Carina Wiese (Franziska Doppler) and Sylvester Groth (Inspector Clausen) appeared together in the series Deutschland 86 (2018), which is set in 1986.