“In 1972 (Garry) Marshall managed to finance a pilot (for Happy Days) by doing a segment for Love, American Style…, starring Ron Howard, Anson Williams, Marion Ross, and Harold Gould. (Eventually the first three would go on to take up their roles in the series.) The segment had enough interest that (then ABC National Programming Director Michael) Eisner supported continued development, and (Happy Days’ other executive producers) convinced Eisner to give series approval in 1973 when they could also plan to use adjacent publicity that (Ron) Howard was gathering from starring in the ‘companion’ film, American Graffiti (Lucas, 1973).”
Staiger, Janet. Blockbuster TV: Must-See Sitcoms in the Network Era. New York and London: New York University Press, 2000. Web. p. 115.
Postscript: Cindy Williams was also a part of this equation, starring as Howard’s girlfriend in American Graffiti, then turning a popular one-episode appearance with Penny Marshall on Happy Days into the spin-off Laverne and Shirley.