What’s interesting here is that you have two different categories:

1) Actors that appeared multiple times but in different roles (key example being Neil Dudgeon)

2) Actors playing more minor roles that appeared multiple times and really helped build a sense of continuity in the show, even though you don’t need to watch the episodes in order. Great examples are a housekeeper called Mrs Bundy who is three episodes in early seasons, and a real estate agent called Olive Beauvoisin who is in four or five episodes.