Is Stephen Colbert the man nobody knows?

With much fanfare, “The Late Show With Stephen Colbert” (10:30 p.m., WREG-TV Channel 3) debuts.

Colbert replaces David Letterman, who once tangled with Jay Leno, who blessed and then usurped his anointed heir, Conan O’Brien, who then withdrew to the land of cable after much wailing and gnashing of teeth. Only after being bloodied by this struggle did Leno acknowledge his weariness and give way to Jimmy Fallon — known as Fallon the bland or Fallon the merry, depending on one’s taste.

CBS has to wonder if Colbert will fit in seamlessly, or merely find a niche audience as O’Brien did during his short “Tonight Show” tenure.

The mystery is compounded by the fact that Colbert’s reputation is based on his performance as the “fake” host of “The Colbert Report.” Who is the “real” Colbert? Will he be funny? And what kind of “funny” works in late night these days?

Part of Fallon’s success is due to his genial personality and his need to be an entertainer. He’s nice, not edgy. ABC’s Jimmy Kimmel also identifies with an every-guy audience. Fallon, Kimmel and CBS’ James Corden have expanded the scope of their audiences by creating bitesize bits suitable for YouTube. BBC America’s Graham Norton also gained popularity in this manner.

These bits obviously require the cooperation of celebrity guests, creating a rather friendly atmosphere on the couch. Has it become too friendly to be interesting?

Part of Letterman’s grouchy charm was to keep some personalities at arm’s length. That’s hard to do when you need them as YouTube co-producers.

On “The Report,” Colbert rarely failed to spoof or create some ironic distance from his guests. That was easier when everybody knew he was playing the fool.

Will that change with Colbert’s network perch, where a need to expand his audience coincides with a “kinder/gentler” comedy era? Like O’Brien, Colbert’s appeal can be seen as too quirky and cerebral. That didn’t work out for O’Brien — at least not on network TV.

Colbert’s first guests, George Clooney and Jeb Bush, bear the stamp of official importance. But edgy they are not. It takes time, years even, for the best hosts to find their way. This should be fun to watch.

TV-themed DVDs available today include “The Hee-Haw Collection” and “Supernatural, Season 10.”

SEASON FINALES

Jeff Foxworthy hosts “Are You Smarter Than a Fifth Grader?” (7 p.m., WHBQ-TV Channel 13).

Girth becomes a professional liability on “Extreme Weight Loss” (9 p.m., WATN-TV Channel 24).

Cedric The Entertainer appears on “Hollywood Game Night” (9 p.m., WMC-TV Channel 5).

Kenny Loggins gueststars on “Playing House” (9 p.m. and 9:30 p.m., USA).

Meaningful games await on “Hard Knocks: Training Camp With the Houston Texans” (9 p.m., HBO).

OTHER HIGHLIGHTS

Eleven acts enter the second part of semifinals on “America’s Got Talent” (7 p.m., WMC-TV Channel 5).

Critters overrun a hospital on a two-hour helping of “Zoo” (8 p.m., WREG-TV Channel 3).

PBS repeats part two of Ken Burns’ 1990 documentary “The Civil War” (8 p.m., WKNO-TV Channel 10).

The new biography series “The BET Life of” (8 p.m., BET) profiles Chris Brown.

A family wake hosts private deals on “Public Morals” (9 p.m., TNT).

Rescuers recall a tragic day on “9/11: The Lost Hero” (9 p.m., Destination America).