That feeling hit me the other night when watching Key Largo. It is kind of odd to see the actors captured on film in their prime but long gone. I did have to look up Lauren Bacall though. I forgot if she had passed or not.
I guess I'm a little morbid, sorry. I try coming up with my own mental obituary list (much like 'The seven degrees of Kevin Bacon') while watching an older movie.
I sometimes get that feeling when watching a rerun of Johnny Carson. I'll think, "Hmmmm, everyone on the show has died - including the host. Some of the guests died rather young.