It also has to do with what their personal publicists and the network/studio publicists think is worth their time. Zac is the face of Chuck, always has been, he seems to enjoy doing the talk show circuit, and his publicist is keen on getting him booked on talk shows; therefore, he’s the one you see most often.

I’m not sure what kind of response a Zac/Ryan or Zac/Vik appearance would generate. But I remember the Zac/Yvonne bit with the golf cart on George Lopez, and what a buzz that created. I’ve seen each of them do solo guest spots, and they are all fine and good, but together they are more than the sum of their parts, which is the very definition of “chemistry”.

What about Scott Krinsky, Vik Sahay, Sarah Lancaster, Adam Baldwin, Ryan Mcpartlin and others ? Shouldn’t all cast members be on late night talk shows ?:)
Not that i don’t love Yvonne or Josh but it’s as appropriate to invite them as it’s appropriate to invite Adam Baldwin or the others