For example, in The Big Bang Theory, could they watch Roseanne? Do Johnny Galecki, Laurie Metcalf, and Sara Gilbert, the actors, exist in the BBT universe?

Also, 30 Rock has mentioned Friends existing as a TV show and Jennifer Aniston as a real person, but also had David Schwimmer and Jennifer Aniston guest star as minor characters. For that matter, Alec Baldwin once guest starred on Friends.