Comic actor John Leguizamo joins the cast of “ER” (9 p.m., NBC) as Dr. Ernesto Clemente, a supervising physician not afraid to use unorthodox means to shake things up. In fact, he will “introduce” himself to the staff, not as a physician, but as a patient, going undercover to see how his staff behaves when their guard is down.

With the introduction of Kirsten Johnston as a tough head nurse, and now Leguizamo, the producers of the long-running hospital soap are clearly not afraid to challenge viewers with strong new characters. The question remains if viewers are ready. You can’t blame the “ER” faithful for suffering from trauma-fatigue. After years of fires, floods, riots, shoot-outs and helicopter blade amputations, it may take more than John Leguizamo to find the “ER” fans who have vanished “Without a Trace.”

¢ Speaking of familiar faces popping up on long-established shows, Richard Chamberlain joins the long parade of guests to appear on “Will & Grace” (7:30 p.m., NBC). Chamberlain appears as an elderly man whom Will invites to dinner, much to Grace’s chagrin.

The star of the “Thorn Birds,” “Shogun” and “Centennial,” among others, Chamberlain reigned as the king of the miniseries when that genre was at its apex. Viewers of a certain vintage will remember him in the title role of “Dr. Kildare.” The “ER” of the Great Frontier era, “Kildare” ran from 1961 to 1966 and established Chamberlain as a handsome romantic lead.

Tonight’s other highlights

¢ Jeff Probst is host on “Survivor: Guatemala” (7 p.m., CBS).

¢ St. Louis and Houston meet in Game Seven of the National League Championship Series (6 p.m., Fox). If a game is not required, Fox will air back-to-back repeats of “Reunion.”

¢ News about Rachel puts Sidney in peril on “Alias” (7 p.m., ABC).

¢ Chris befriends a cool kid on “Everybody Hates Chris” (7 p.m., UPN).

¢ A boy and his apparently abusive Dad leave a trail of homicide mysteries on “Night Stalker” (8 p.m., ABC).

¢ A missing student, long thought dead, may still be alive, on “Without a Trace” (9 p.m., CBS).

¢ A woman suffers hallucinations in a mad scientist’s house in the 1962 shocker “The Cabinet of Caligari” (9 p.m., Fox Movie Channel), based on a screenplay by Robert Bloch (“Psycho”).