Kaley Cuoco, who plays Penny in "The Big Bang Theory," revealed on "The Ellen Degeneres Show" that Penny's transition from waitress to bartender in Season 4 was due to a real-life horse riding accident she had. The accident left her with a broken leg and forced her to miss a few episodes of the show.
To hide her injury, the show's writers put her behind the bar instead of waitressing, giving Penny some new comedic opportunities. Cuoco returned to the show wearing a full leg cast and continued to play Penny as a bartender instead of a waitress. This change allowed Penny to establish new comedic bits and adopt a bartender-as-therapist persona.