He had it down from the very beginning.

I know I've said this about a later episode, but I'll say it here, too, Peter Falk was born to play Columbo. Even in this first Falk pilot, "Prescription Murder", made in 1968, several years before "Columbo" became a series, Peter Falk had this character 99% perfect in this first go-around as the raincoated LAPD detective.