A short video from PBS about the Golden Years of Radio
A short film from 1938 demonstrating techniques used during radio plays of the period.
Dean Martin and Orson Welles demonstrate early foley techniques.
An excellent case study of radio stardom is the career of actor William Conrad, who exhibited the great potential and impact of the best actors radio drama had to offer. Notably, many radio stars occupied a definite place in the public consciousness, with 82% of Americans identifying as radio listeners in 1947 according to a C. E. Hooper survey. Besides the potential for fame, radio stars also had the opportunity for enormous agency in the radio industry. William Conrad, for instance, was already producing in addition to performing in the early days of his career, even before World War II.
However, even the biggest stars of radio drama were not immune from the historical machinations of the 20th century. Initially during his service in WWII, Conrad served as a fighter pilot in the United States Armed Forces before night blindness grounded him into the Armed Forces Radio Service.
But radio drama did have many benefits, including giving actors the ability to work at a considerably fast speed. In the immediate years after WWII, William Conrad performed hundreds of roles in dozens of radio series, providing a variety of material for him to engage with creatively. The form demanded from and cultivated in the actors who worked in it specific skill sets. Some of these performers successfully made the transition out of the Golden Age of Radio (roughly 1930-1950) into the rise of television, such as husband and wife comedy team George Burns & Gracie Allen (who had the benefit of preexisting film careers). Others, such as Fred Allen, did not.
Source: https://www.britannica.com/topic/Golden-Age-of-American-radio
Source: http://www.radioclassics.com/william-conrad-the-voice-of-postwar-radio/
Source: https://www.britannica.com/topic/radio/American-radio-goes-to-war#ref1123988
Source: https://www.sparkmuseum.org/the-golden-age-of-radio/