Radio drama uses sound to convey ideas to the audience. The sound may be in the form of dialogue, sound effects, or music.
Actors can play more than one part from moment to moment as only voice needs to be altered.
Actors use voice alone to convey character. They do not need to use action, gesture, or facial expression as the audience cannot see it.
There is no need for blocking or stage business in a radio drama. Actors may however need to plan movement to and from the microphone.
Radio plays can be set almost anywhere. There are no physical limits to be overcome. Setting is easily created using music and sound effects.
Radio drama includes lots of dialogue. As we cannot see what the actors are doing, they must tell us what is happening. The writer has to find ways for the characters to comments on where they are and what they and others are doing.
Radio play scenes are often shorter that stage plays.
Radio drama may not be 'live' in front of an audience. A radio drama is usually recorded and played to the audience at another time. This means that, like a film, scenes may not be played in order and scenes can be played more than once to get the best 'take' if necessary. The drama can also be edited to add in sound.
Actors don't have to learn their lines. Actors can read their lines from the script as this is not seen by the audience. Actors do have to be very familiar with their lines and to have rehearsed how they will use their voice to tell the audience about the character and the action.
Radio drama usually features a small cast. Small numbers of characters are used in each scene because it is too hard for the audience to distinguish between voices if there are a large number of characters. The audience may also forget that a character is 'on stage' if he/she does not speak for some time as they cannot be seen. For this reason characters need to speak quite often and be involved in the action.
Radio drama is well suited to monologue and for plays where setting is limited and physical action is unimportant for example inside a prison cell or a cave.