I totally agree Irene...and there are many little moments like that...where Falk's acting makes these small tidbits stellar.
I love when Friend in Deed...Columbo is at Halperin's house, consoling him after the 'drowning" and says to the Commissioner...as he pats his chest..."you mustn't blame yourself sir"
I also love Columbo's facial reaction to being chewed out by Riley's lawyer in Publish or Persish...in Riley's driveway..."Evidence Lieutenant. find the evidence" etc...

absolutely, those little moments make the show very enjoyable to watch.

One of my favorites is in Prescription Murder, at the very end when the killer realizes Columbo's trick, he looks over at Columbo and Columbo gives him a look that I don't even know how to describe, but it's perfect.

There's no one thing that keeps me interested. It's a combination of things. Speaking primarily of the early episodes:

Terrific plots.
The singular atmosphere of the 70's.
The Americana.
The truly 'lived in' feel and contrast between scenery, from upscale and regal to the dark and gritty.
Realistic dialogue and nuanced performances from a variety of intriguing characters.
The warmer picture quality and cinematography.
The soundtracks.
And of course the man himself, Peter Falk.

Everything just worked brilliantly.