Leslie Nielsen playing the incredibly young captain in Forbidden Planet.
LTS, if you look in the credits to Undiscovered Country you will see that the casting director is Mary Jo Slater--Christian's Mom. It helps to have connections!
As for Folkies, A.L. Lloyd is the Shantyman in Moby Dick, and Barry Dransfield played a blind fiddler/singer in The Bounty (w/ Mel Gibson and Anthony Hopkins). Damien Barber turned up on something I was watching--could it be Lovejoy? Tom Clancy played a jet plane mechanic in an awful TV movie about a haunted airplane, starring Ernest Borgnine. In Cold Mountain Renee Zellweger's baby is played by Tim Eriksen's baby--Tim was a musical consultant to the film. (Not that you'll go "hey, isn't that Tim Eriksen's Baby?")
By the way, in that first Death Wish movie (which was filmed in MY old neighborhood), another of the muggers is the guy who played Freddie "Boom Boom" Washington on Welcome Back Kotter. (Speaking of which, a pre-stardom John Travolta was on that show.) The first mugging takes place at the 79th street Boat Basin, where I hung out as a kid, and the wife and daughter are followed home from a D'Agostino's where we used to shop. If you come from New York City, you get used to that. I went to Columbia, and saw not only the buildings where I took classes but also the little nook that housed the tutoring office where I worked, in Spider-Man 2. And Ghostbusters. And several other flicks.
Now I live in Philadelphia, and I can see why people remember Rocky, Philadelphia and Trading Places so much; not that many movies get made here!