These Celebs Guest-Starred On Television Shows Before Their Big Break

Millie Bobby Brown in "Grey's Anatomy"

Before Eleven, she was Ruby, a patient at Seattle Grace Hospital. Her mother fell and hit her head, and Dr. Owen Hunt helped walk her through a procedure over the phone until help could arrive. 

Bradley Cooper in "Sex and the City"

Bradley Cooper's first credited role was in the hit HBO show, "Sex and the City." He plays a jerk — shocker — that Carrie meets on a night out. They don't hit it off, and he didn't return for another episode. It served as a great launch point for his career, though.