I only want to add that I saw all Columbos dozen of times, but in my country is always with a lector, it always bothered me, I've heard original talking but the lector was disturbing it, now I ordered DVDs when I finaly can hear original as it is. Pure pleasure, it's like I was discovering it again, and now I see all this little things in the bacground. Amazing
Columbo does have some great background, and even foreground, characters. My favorites are the people that dancing to the funky music at the pool in Any Old Port in a Storm.
I don't know if you consider these secondary characters but in "Identity Crisis" and "Catch Me If You Can," I like the reaction Columbo has to the belly dancer's. It's amusing.
Something that slightly bothers me is when Bruce Kirby plays a background character in one episode like the lab attendant in 'Lovely but Lethal' but then plays a Police Sgt. in another episode.