Watching Columbo reruns I noticed that a lot of actors have repeat performances in different episodes portraying different characters. This page has a big list of such actors, one of them played as many as 19 different roles. It's slightly distracting seeing the same actor again in a different role.
Why did they reuse actors instead of casting new people to fill the roles? Surely it wouldn't have been hard to find more than enough eager actors in Hollywood.