You Know The Voices: Marvin Miller & Paul Frees (1958)

 From season 5 of The Millionaire:

Paul Frees, the voice behind the title character, John Beresford Tipton, essays a dual role in this episode, stepping away from the microphone to play a friend of "The Thorne Sisters". Paul appears at the 13:25 mark. Marvin Miller, in his role as Tipton's personal secretary, Michael Anthony, paints the picture.....

Howard McNear, radio's Doc Adams on Gunsmoke, would land the defining role of his career 2 years later when he was cast on The Andy Griffith Show.