I am looking forward to writing a radio serial, but have not heard too many old time radio shows yet. (About the only ones in my personal collection are the Basil Rathbone Sherlock Holmes adventures.)

Question: Just how much role does the announcer serve? Can I use him or her as a sort of Narrator? ("Our heroine had a bad feeling about this---the ancient building seemed to be ready to collapse under her sneakers at any moment..." ) Can I use the announcer to hint at my MC's motives and/or thoughts? Or are they just there to do the usual, "Welcome back to this week's adventures of The Shadow! Who knows what evil lurks in the hearts of men...blah blah blah...When last we left our intrepid hero...." and the usual end of episode cliff hangers?

I know so much of radio depends on sound effects and dialogue--(by the characters themselves) and dramatic music--guess I am just trying to figure out how much.

Normally, as a writer you don't have any influence on how your audio play will be announced on the air, so you cannot depend on the announcer to convey information the listener actually needs.

Check out BBC7:
http://www.bbc.co.uk/bbc7/programmes/a-z/player

They always have loads and loads of audio drama which you can listen to online and normally you will also hear the announcement.

And acually, I think you shouldn't need to hint at the feelings of your heroine at all, neither with a classic narrator nor with any kind of outside explanation. Listeners are much more intelligent than you might think. They will get it! Trust them.

What you can do to get the listener back on track for a sequel is to repeat the last few minutes of the previous ep. That's how BigFinish does it with their Doctor Who dramas (where the individual eps normally end with a huge cliffhanger). It is a bit annoying if you listen to the eps in direct succession (like you do when you have them on CD), but it is great for radio when there is a week in between eps.

Announcer and narrator are NEVER the same person.
Announcer is the person who speaks the announcement before the play is aired, most of the times that's done live in the program. Narrator is part of the play, and is recorded on the tape or file.