NU's School of Speech and its theatre program has produced such famous
entertainers as
Edgar Bergen,
Charlton Heston and
Ann-Margret.
Not everyone on this list graduated, but theatre majors like to be inclusive because they'd like to add their name to this list someday, and the alumni magazine knows celebs make for better covers than engineers. Not everyone on the list is equally famous, of course, (you'd have to watch a lot of American television to pick out some of these names) but with the exception of a few up and coming Broadway stars and one MacArthur fellow (Zimmerman), you should find info on all these folks at the imdb.
Other Northwestern alums featured on the small and big screen, such as Ana Gasteyer, Seth Meyers, Megan Mullally, Zach Braff, and Amanda Congdon, did not major in theatre.
Cindy Crawford sometimes makes the list. She's certainly famous, but she was only at NU a semester, and tradition has it she majored in engineering.