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]