Lucy and Ethel were the first of television's best girlfriends, making audiences laugh long after "I Love Lucy" left the airwaves. In real life, it was a friendship that lasted decades, and what you saw on-screen often echoed what was happening off-screen. Lucille Ball and Vivian Vance met when their legendary sitcom began, and a lifelong friendship was born. In 1951, Ball and Arnaz began looking for performers to play their two best friends, a couple named Fred and Ethel Mertz who owned the Manhattan building where Lucy and Ricky Ricardo lived in. That's when Ball met Vivian Vance, a former theater actor who had appeared in a few movies without yet finding her fortune and fame (via The Famous People). Ball recalled in her autobiography, "Love, Lucy," how she and Vance felt like friends almost immediately.

Source: The List