Martin Sheen appeared on a couple of episodes of Garrison Keillor's "A 
Prairie Home Companion" this year - I only just found out!
The shows are available online here: 
http://prairiehome.publicradio.org/programs/2007/06/09/ and 
http://prairiehome.publicradio.org/programs/2007/09/29/ (both are show pages 
that contain links to the streaming files - you need Realplayer or 
compatable player!)  :)
Au Res.,
Paul