Parts of the interview are, but the majority of it is Stephen.
In essence, the pre-interview, before the show, Stephen will meet the guest and chat him or her up for a few moments and also tell them not to take him too seriously and if he offends them.. tough.
It should also be pointed out: Jon is an Exec Producer of The Colbert Report, so a lot of what goes on in that show stems from Jon as much as from Stephen.
Take care all.