Two points I should have made in my two previous posts on this…
I said I find Craig Ferguson to be the most spontaneous and unrehearsed talk show host on at the moment. A lot of this is probably because he doesn't usually book as guests, people who have a big $100 million dollar (and up) movie opening next week or a brand-new series debuting next week. This may be because he doesn't want such people but it's more likely because Dave Letterman, who owns the show, doesn't want that. Letterman has always put certain restrictions on the show after his just as, back when he followed Carson, Johnny decreed that Dave's opening monologue could only be so long, the band couldn't be too large, etc.
If Craig did have on a guest with a huge movie opening, there'd be a lot more pressure — from the movie studio if not from the star's handlers — to control the interview, to make sure the star scored big and got to say all the right things leading in and out of the clip, etc.
Other point…
Conan O'Brien and Jimmy Fallon differ from their Tonight Show predecessors in one key way that I think gets overlooked and which may account for a certain amount of difficulty in getting established. They're the first two Tonight Show hosts who had to set up shop in that time slot and suddenly appear right after a popular guy left.
Jack Paar did not follow Steve Allen. When Steve Allen (and Ernie Kovacs who by then was hosting two nights a week) left Tonight, Paar was not their replacement. NBC tried a new show — a switch on the Today Show format called Tonight: America After Dark. It was a mess with a gang of correspondents interviewing different people in different locations and it was a ratings disaster. That's what Paar followed into that time slot — a show no one was watching.
Johnny Carson did not follow Jack Paar. Carson was still under contract to ABC to do the game show, Who Do You Trust?, and they wouldn't let him out right away. What followed Paar was The Tonight Show with rotating hosts — the first was Art Linkletter — while they waited for Carson to be free of his afternoon commitment. The interim show was another mess that had no real following. Johnny followed that into the time slot.
Jay Leno did follow Johnny Carson but with a key difference: Jay had been Johnny's guest host and at times, was hosting the show as often as Carson. He was darn near the co-host, doing every Monday night and sometimes an entire week or two. Johnny did his last broadcast on a Friday night. Jay did his first the following Monday…but that wasn't a jarring change for America. There wasn't someone brand-new on NBC at 11:35. Jay would have hosted that night even if Johnny hadn't left. He might have hosted that entire week.
Conan O'Brien was the first Tonight Show host whose presence at that hour seemed to be immediately displacing someone America had grown used-to at that time. Jimmy Fallon is the second…and this is the first time they've ever tried to establish a new host at 11:35 and almost simultaneously (like, beginning next week) tried to launch a new show at 12:35. In Prime Time, the odds of coming up with two new hit once-a-week sitcoms back-to-back are pretty remote. Let's see how they do with two five-nights-a-week hour shows.