Corrected entry: Peter absorbs René "The Haitian"'s abilities (erase memory and neutralize abilities). Later on he fights Sylar and uses René's ability to force Sylar out. Sylar faints and turns into Nathan. If Peter absorbed René's abilities to neutralize abilities, shouldn't Sylar be unable to shape-shift into Nathan's looks?
Corrected entry: Sylar takes over Parkman's body, and has discussions with Parkman, who is a figment and isn't really there. However there are many times when a shadow is cast on Parkman. Most noticeable when they are in the diner in Texas and Sylar threatens to kill Lynette, and Parkman interjects (when Sylar instead takes a straw). You can see Sylar's shadow on Parkman, which isn't possible if he's not really there.