Since 2000, Cassidy has appeared in the film "Ghosts of Mars" (2001) directed by John Carpenter, and had a recurring role as Margaret Chenowith on the HBO drama series "Six Feet Under", for which she received an Emmy Award nomination. In 2004, she guest-starred in two episodes of ' as T'Les (the Vulcan mother of "Enterprise" crewmember T'Pol), and she had a recurring role as Beverly Bridge on the series "Boston Legal" in 2006. She voiced the villainess, Hecubah, in the computer game "Nox" (2000) and once again voiced the character of Maggie Sawyer in the 2002 video game '.