In some of the episodes, Columbo does little things which makes me wonder if they were ad libbed. When I watch, I look for them because it makes the episodes more entertaining. Examples:
"Now You See Him" - When the Great Santani gets out of the handcuffs, Columbo winks at him implying "I got you."
"Forgotten Lady" - He throws a kiss to the other detective thanking him for taking the his gun test.
"Murder Under Glass" - I don't know if this is the correct episode but he talks about how he was a Chef during the war. Which is true because he was a chef in the Merchant Marines.
Does anybody have other examples?
I believe he and Martin Landau ad-libbed most of that cooking scene in "Double Shock".
Although falling down the hill in "The Greenhouse Jungle" wasn't really an ad lib (more like an accident), the following dialogue between Columbo and Sgt. Wilson was a pretty good ad lib.
Columbo: "That's a little steep. But I tell ya' it was the quickest way down."
Wilson: "Oh yes sir, by far".
And then Wilson helps him up and brushes him off and Columbo gives him a funny look.
I think all ad-libbed.
While there are some obvious ones, much of the genius of the show and Falk is that you can't often tell when he does it or not. Bochco explains it so well here at the 7-minute mark: https://www.youtube.com/watch?v=O2wOnLS7lqc
I'll keep looking for them though...
Almost every actor in Last Salute For The Commodore seems to be doing a little.