Joshua Malina as President Siebert
President Siebert was played by Joshua Malina. Siebert presided over CalTech for the duration of the series. Siebert and Sheldon had a running gag where Sheldon would frequently visit or call Siebert in the middle of the night for information.
Brian Thomas Smith as Zack Johnson
Zack Johnson, played by Brian Thomas Smith, was a dim-witted but well-meaning jock-type who dated Penny a myriad of times after her second break-up with Leonard. Zack's stupidity was off-putting to Penny, often driving her back into the arms of Leonard. Brian Thomas Smith has appeared in many commercials over the years for popular brands like State Farm, Chevrolet, Taco Bell, and Pizza Hut.