I'm semi-addicted to watching 'The Avengers' at 8pm on weekdays. (True Entertainment, Channel 61 on Freeview). In many episodes every one of the subsidiary actors is someone who later had a leading role in some other TV series.
Things can get confusing though when the same actor plays more than one role in a series. Some time ago I turned on an old episode of Last of the Summer Wine (on GOLD) to be confused by 'Tom Simmonite' appearing dressed much more smartly than usual and without his usual accent. Then Compo appeared and I realised that Tom Owen (who later played Tom Simmonite) had appeared much earlier in the series (in the episode I was watching) simply as 'Man at Cash Machine'.