David Letterman may be unable to follow up last night's John McCain appearance with one from his vice presidential running mate, but at least he's got the next best thing: Tina Fey! The 30 Rock actress has already taped her guest spot on tonight's Late Show , and we have this clip where she breaks down her Sarah Palin impression . So what exactly are her influences?Turns out, it's "a little bit Fargo , a little bit Reese Witherspoon in Election ," with just a soupçon of her friend Paula's grandma from Joliet, Illinois. Fey downplays her frightening accuracy by claiming it's the easiest impression to do since Billy Bob Thornton mmm-hmmed his way through Sling Blade , but we have to give credit where credit is due. Now, Tina, where the hell is 30 Rock ? Can't you pull some strings and get Palin to fire Ben Silverman ? [CBS ]