In the opening TARDIS scene, Polly, Ben and the Doctor are each heard "thinking" about where they would like to land next in a rare example of internal monologue. This was achieved by pre-recording the actors' voices and playing them back during the making of the episode.
A surviving production still image, used in the reconstructed DVD release, reveals the note hand-written to Zaroff in which the Doctor signed as "Dr. W", a very early indication of there being more to the Doctor's name than just his title.
Only episodes 2 and 3 of this story exist with episode 2 representing the earliest existing episode from the Patrick Troughton era. This story was released on DVD using production stills and existing audio for the missing episodes 1 and 4.
This story was written before it was known that Jamie would become a main character. His lines were shared with Ben.
In the original script, Professor Zaroff had more motivation for his scheme - he was driven mad by the deaths of his wife and children in a car crash.