Look for film critic Tom Shales when those thumbs go up or down on TV next weekend. Shales will be the first revolving co-host to fill in for Gene Siskel on the syndicated “Siskel & Ebert” show while Siskel (he’s the skinny, balding one; Roger Ebert’s the pudgy one) stays home to further his convalescence from brain surgery. Shales, like Ebert, is a Pulitzer Prize-winning critic. He is The Washington Post’s TV critic but has reviewed movies for the Post and National Public Radio (and, like Ebert, he’s pudgy, too). Siskel, the film critic for the Chicago Tribune, had surgery in May to remove a growth on his brain. Ebert, the film critic for the Chicago Sun-Times, has been going it solo since Siskel took a leave of absence Feb. 3.