The thin line between reality and TV blurred even further yesterday when Homeland Security Secretary Michael Chertoff dropped by the “24″ set to meet Kiefer Sutherland. Chertoff, a huge fan of the show, got his picture taken with the actor and watched Jack Bauer and FBI agent Renee Walker (Annie Wersching) repeatedly drive up to the Willard Hotel back entrance and go inside — presumably to find bad guys. No Chertoff cameo? “I think he was happy to observe,” Fox’s Chris Alexander told us. After Chertoff left, Sutherland (blue shirt, black slacks, khaki jacket) spent downtime in a tent (playing chess and smoking) and chatting with the crew.