Answer: Simon Pegg
As well as starring in the film, Simon also co-wrote the script with director Edgar Wright. The two had previously worked together on the British sitcom, "Spaced", which Simon had starred in and had co-written with Jessica Stevenson, who appears in this film in a minor but important role.
Nick Frost played Simon's best friend Mike in "Spaced" and appears in "Shaun of the Dead" as his best friend Ed. Dylan Moran plays David and Peter Serafinowicz (the voice of Darth Maul in "The Phantom Menace") plays Shaun's uptight flatmate, Pete.