Just came across this one -- Columbo (in costume, more or less, and barely in character as Peter Falk) on Carson, talking about "Exercise in Fatality" and hinting about how fantastic the key clue is. Peter and Johnny having a good time. There seems to be some kind of inside joke going on, when Peter turns his backside toward Johnny -- see what you think:

https://www.youtube.com/watch?v=6SC4Ae6FuWM

He seems pretty relaxed and at ease with columbo...Pity he didnt guest star in a couple of episodes.

Johnny did make an "appearance" in "Forgotten Lady".