At I discovered girls. At that time, dancing was the only way you could put your arm around the girl. Dancing was courtship. Only later did I discover that you dance joy. You dance love. You dance dreams. ~ Gene Kelly, Singing In The Rain
Donald O'Connor had the great dance skills and the comedic timing to make him one of the best ever filmed. Best evidence is Moses Supposes with Gene Kelly and Make 'Em Laugh both from Singin' In The Rain.