I think his acting changed as he got older. Bearing in mind he started playing Columbo when he was 41 and was 51 when the '60s and '70s episodes finished, I think he was growing tired of the role. (Of course he then reprised it later in life).

Whereas the early episodes have him almost school boyish and far more active - running here and there - the later ones have him more thoughtful and contemplative, with scenes showing him deep in thought (e.g. "Try and Catch Me" where he is sitting staring at the walk-in safe, or "Old Fashioned Murder" when he is sitting contemplating the room, while the suspect is upstairs).

Studying 'The Columbo File' actually may shed some light on things. The added presence of Richard Alan Simmons as producer does tend to change the feel of the series a little from 'The Bye Bye Sky High IQ Murder Case' onwards. In a sense, the show has a slightly more psychological feel, as in delving a little deeper into the emotional make up of the killer, which perhaps did give a different dimension to the show, amongst a few other subtle changes. The previous season - season 5 - had shown a few dabblings with the familiar formula, too. 'Forgotten Lady' showed the killer being let off, and 'Identity Crisis' seemed rather too complicated for its own good (especially if only viewed once), in addition to 'A Case Of Immunity' touching on middle eastern politics. The biggest departure of course being 'Last Salute To The Commodore'. Only with 'A Matter Of Honor' and 'Now You See Him' did the series seem fully on more familiar territory. However, Falk's portrayal still doesn't seem so hugely different, even judging by the off beat 'Last Salute To The Commodore'.

Yes, "Last Salute..." was very strange. I think this certainly had the most flimsy "Columbo moment" with Swanny saying "T'isn't!" to the sound of the ticking watch.

As for "Forgotten Lady" it wasn't really a case of letting her off. Her medical record showed she only had a month or two left to live, so it probably wouldn't have even got to trial - plus of course she had no recollection of murdering her husband.

Some of them, like "Identity..." did seem to try something different, but didn't necessarily work

I think Identity Crisis is pretty solid. It does, along with A Case Of Immunity, introduce the notion of Columbo being thwarted, albeit momentarily, by bureaucracy. Perhaps it was or wasn't the case back then but that's definitely a rather cliched obstacle for "maverick" detectives on TV and in film these days.

Crucially though, in each case, it's more than the usual "I'll have your badge" type threats and it's how Columbo overcomes this which is vivid and consistent with character. In Identity Crisis, Brenner brings his CIA boss down on Columbo. Instead of being intimidated though, Columbo uses this intervention to confirm what he already suspected about the state that the victim's body and the likely assailant. Brenner even recognises this as the key to Columbo's tenacity which, as a fellow warrior perhaps, the double agent admires.

In A Case Of Immunity, Columbo uses his judge of character and intel gleaned from the protesters in relation to both Hassan and the King to devise an audacious sting. First of all, the King's open mindedness in allowing the charade and, of course, Hassan's hubris in being protected by diplomatic immunity and the belief that Columbo might give up on a murderer for the sake of his job. This bears fruit when Hassan can't wait to deliver himself into the Lt.'s custody and compliments him by not even reading the confession which he has already prepared. I love also how Columbo is as coy about accepting Hassan's plea for relative sanctuary in the arms of the LAPD as he normally is about accusing or implicating anyone.

Another excellent example of this is "Friend in Deed" when it's the Deputy Police Commissioner who's the suspect.

I mean he gives himself away when he refuses to consider the clues Columbo comes up with about the fingerprints on the phone and the night-dress.

He constantly orders Columbo to concentrate on it being a burglary gone wrong, and then Columbo actually uses the burglar to help frame the Commissioner.