Before they were Hollywood stars, Steven Yeun, Octavia Spencer, and Kate Micucci had memorable guest roles on The Big Bang Theory.   From Yeun's appearance as Sebastian in "The Staircase Implementation" to Spencer's hilarious stint as a DMV employee in "The Euclid Alternative," these performances were standout moments that hinted at their future success.   Let's watch the video to see these unforgettable cameos...