Character actor Karen has had a 40-year career as an actor. He made his Broadway bow with Marlon Brando in A Streetcar Named Desire, directed by Elia Kazan. Since then, he has worked continuously in theater, television and film, with such greats as his idol Buster Keaton, and on up to director Oliver Stone. His best-known films include Return of the Living Dead (1985) and Return of the Living Dead II (1988). Karen has also appeared in All the President's Men (1976), China Syndrome (1979), Poltergeist (1982), and Wall Street (1987). He was a regular on Eight is Enough (1977-81), starred in the science fiction series The Powers of Matthew Star as Major Wymore (1983) and had a recurring role on the cable series The Larry Sanders Show. Karen took on a series of small roles in notable films throughout the early 2000s; among his credits include Any Given Sunday (1999), Thirteen Days (2000), and Mulholland Dr. (2001). He played a supporting role alongside Will Smith and Thandie Newton in the 2006 drama The Pursuit of Happyness, and appeared in Superman Returns the same year. He worked with Chevy Chase and Christopher Lloyd in director Gary J. Tunnicliffe's adaptation of Jack and the Beanstalk in 2009, and took a small part in 2010's psychological drama Sympathy for Delicious.
- Served in the United States Air Force during World War II.
- Made his Broadway debut in the original production of A Streetcar Named Desire in 1947.
- Appeared in the 1965 Buster Keaton movie Film; received the Buster Award from the International Buster Keaton Society in 2001 for his close professional and personal friendships with Buster and Eleanor Keaton.
- In 1968, he became a spokesperson for Pathmark supermarkets.
- Presented with a Life Career Award by the Academy of Science Fiction, Fantasy and Horror Films in 1998.
- Had a role as Martha Kent's friend in Superman Returns (2006), but his scenes were edited out of the film's theatrical release.
- Touched on his character-actor status in a 2006 interview about the DVD release of the cult hit Frankenstein Meets the Space Monster (1965), explaining, "It was the first and last time I played the lead in a picture."