I think many of the guests who come on the show to be interviewed come off looking great. Most recently, former New York Times editor Howell Raines, who was interviewed on Monday. I thought he seemed very sweet and genuine. Julian Bond of the NAACP was on last week and did an excellent job.
On the other hand, the Congresspersons who are interviewed in the Better Know A District segment are intentionally made to look like buffoons. They are asked ridiculous leading questions, and the interviews are edited to be as silly as possible. Although Wexler was clearly going along with Colbert’s joke, I thought while watching his interview that he would have done a lot better to point out that even if you are in an uncontested election, discussing cocaine use and prostitution could be detrimental in other ways. I was shocked that he didn’t answer the prostitution question with some variant on, “it may not matter to the voters, but it will matter to my wife.”
I think that the best a Congressperson can do is to watch the show beforehand to get a feel for it (and it does help to know some of the hundreds of inside jokes), answer questions genuinely, have a sense of humor, and follow the SDMB rule, don’t be jerk.