The project, based on an Israeli series of the same name, stars Gabriel Byrne ("The Usual Suspects") as a therapist who's great with patients but a wreck with his own shrink. George and Underwood will each play patients of Byrne's, according to The Hollywood Reporter.
Additionally, Dianne Wiest ("Law & Order," "Bullets Over Broadway") has signed on to play Byrne's mentor, with whom he schedules a therapy session years after dropping her.
"In Treatment," which is executive produced by Mark Wahlberg ("Entourage"), among others, has a five-episode order from HBO -- but not a commitment from the network to put those episodes on the air. Instead, the premium-cable channel is treating the episodes as an extended pilot.
Underwood, a former "L.A. Law" regular, starred in the Tyler Perry movie "Madea's Family Reunion" earlier this year. His recent small-screen credits include a recurring part on "Sex and the City," NBC's short-lived "LAX" and a guest appearance on "The New Adventures of Old Christine" earlier this season.
George is best known in this country for her stint as the villainous Lauren Reed on "Alias." She also appeared in last year's "Amityville Horror" remake and "Derailed," and is currently working on the horror film "30 Days of Night," due for release next year.