In addition to his famous role as Jeff Miller in the "Lassie" television series on the CBS network, Rettig also appeared in 17 feature films, including "So Big", "The 5,000 Fingers of Dr. T.", written by Dr. Seuss, and "River of No Return" with Marilyn Monroe and Robert Mitchum. It was his work with a dog in "The 5000 Fingers Of Dr. T." that led animal trainer Rudd Weatherwax to urge him to audition for the "Lassie" role, for which Weatherwax supplied the collie.