::: Both Henry Winkler and Donny Most were credited in Co-Starring roles for Season One; their credits appeared first during the closing titles, indicating that they were considered more as recurring characters.  In Season Two, however, their billings changed and they were now included in the opening titles as fourth and fifth leads respectively, and as a result their characters became more central to the show.  It was the Third Season that saw Winkler's billing change again from fourth lead to second lead, after Ron Howard.