The roles of Fred MacMurray's three sons were all carefully cast, with as much an eye toward their on-camera personalities and their off-screen charisma. Don Grady later recalled "I was very serious. Dramas were all I had done. Since 'Sons' was a comedy, I thought I had to be funny. Peter Tewksbury, our first director, took me aside and said, 'Keep your seriousness…that's when comedy works the best.' I've been serious ever since!" Interestingly, Don was actually considered a "replacement Robbie". The first actor selected to play the role was actually a child actor named Billy Chapin. But MacMurray apparently didn't like him. The show's director Peter Tewksbury had just come from directing "Father Knows Best" (which co-starred Chapin's younger sister Lauren) and it is possible it was his idea to bring Billy on board. But from all reports he was a little too rebellious a kid and MacMurray wouldn't have that.