The 'class clown' from the racy BBC sitcom discusses his return to the stage, the draw of the dark side and preparing his parents to see him in 'full bum' nudity on screen For the past five weeks, Joshua McGuire has been in a whitewashed room in north London pretending to be a rhinoceros. The 37-year-old actor isn't in a performance art piece or strange social experiment, but rather starring in director Omar Elerian'…