 For the "Make Em Laugh" number, Gene Kelly (I) (qv) asked Donald  OConnor (qv) to revive a trick he had done as a young dancer,  running up a wall and completing a somersault. The number was so  physically taxing that OConnor, who was smoking four packs of  cigarettes a day at the time, went to bed (or may have been  hospitalized, depending on the source) for a week after its  completion, suffering from exhaustion and painful carpet burns.  Unfortunately, an accident ruined all of the initial footage, so  after a brief rest, OConnor, ever the professional, agreed to do  the difficult number all over again.