LOS ANGELES (AP) -- What a difference four years have made.
When the cast of "The Big Bang Theory" first appeared at PaleyFest in 2009, their sitcom wasn't even among the top-40 rated shows in the United States.
Last week's first-run "Big Bang" episode topped the list in viewers most coveted by advertisers, those ages 18-49; and it was the second most-watched show overall (behind "NCIS") with more than 20 million viewers.
What made "Bang" so much bigger? Credit syndication. Reruns of the show have been almost impossible to avoid since local stations and cable network TBS debuted them in September 2011 — introducing the series and hooking millions of viewers who didn't catch episodes the first times around on CBS.
"You're kind of forced to watch the show now," joked actor Simon Helberg, one of the show's stars, Wednesday night on the PaleyFest arrivals line. "We're shoving it down people's throats, and then they're learning that they love that."
From co-creators Chuck Lorre and Bill Prady ("Dharma & Greg"), "The Big Bang Theory" spins around two roommates, physicists Leonard Hofstadter (played by Johnny Galecki) and Sheldon Cooper (Jim Parson), as well as their friends and colleagues, engineer Howard Wolowitz (Simon Helberg) and astrophysicist Raj Koothrappali (Kunal Nayyar).
Parsons may play a genius, but he's having a tough time getting his head around the show's current success.
"There's something impossible to fathom about viewers and numbers and things like that," Parsons explained. "I can barely picture 20 people in one room. If you want to say 17 or 18 million, well you've lost me."
The "Big Bang" cast and crew were careful to withhold any major upcoming plot developments, except for one — to be revealed in the first-run episode airing Thursday night: the discovery of a letter from Howard's long-lost father. "They (Howard and his scientist wife Bernadette) find the letter," revealed actress Melissa Rauch, who plays Bernadette. "Or someone finds the letter," Rauch added, coyly.
No other cast members or co-creator Lorre would go near discussions of the characters' futures. But Helberg did agree to dream up a season-20 endgame for the "Big Bang" bunch.
"Well I think (Howard's) pants will probably be as skinny and probably a bit tighter," Helberg began. "I think that Sheldon will probably still be sitting in his spot. It may be in a different apartment. I think he'll sit in that spot until he's in a home. Howard and Bernadette probably will ... God, God help them if they have children. We know she hates them and we know that Howard, essentially, is a child. So I wish them luck."
PaleyFest, an annual open-to-the-public series of TV-series cast and crew panel discussions, runs through Friday at the Saban Theatre in Los Angeles.
CBS is a division of CBS Corp.; TBS is owned by Time Warner Inc.
Follow Michael Cidoni Lennox on Twitter at http://twitter.com/MikeCLennox