6. The X-Files
In the second season episode, "F. Emasculata" Dean Norris pops up as (you guessed it) a law enforcement officer who looks exactly the same as every other cop he has played. Mulder and Scully are sent to a prison to participate in the retrieval of two escaped convicts. Initially they are not sure why they have been sent on what should be a routine police investigation. But once they start looking a little deeper, they find that the inmates are suffering from a mysterious, deadly disease. Norris plays US Marshal Tapia and has his toes firmly stepped on when Mulder and Scully turn up on the scene;
US Marshal Tapia: "FBI run out of crooked politician to sting?" Mulder: "Excuse me?" US Marshal Tapia: "Mulder and Scully?" Scully: "That right. We have orders to work with the Federal Marshals on this man hunt." US Marshal Tapia: "Either of you ever run an escaped convict operation?" Mulder: "No." US Marshal Tapia: "Well then you'd be a real big help if you just tried to... stay out of the way." Mulder: "We'd be happy to, soon as we can talk to someone who's in charge." US Marshal Tapia: "I'm in charge here." Mulder: "Apparently not, or you'd know why our involvement was requested."