"Family Tree" (HBO, premieres Sunday). Christopher Guest has made you a TV series. Thank him. The director of "A Mighty Wind" and "Best in Show" and one of the forces behind and in "This Is Spinal Tap" -- in which he was Nigel Tufnel, whose amplifier went to 11 and whose guitar you were not to touch or even to look at -- Guest has been an architect of modern comedy, from the improvised dialogue that marks his films to the documentary style in which most have been shot. Its sound is his sound, its look his look. (Ricky Gervais owes him his career, if we are to consider that career based on "The Office"; "Parks & Recreation" could almost be Guest's own work.) In the wonderful "Family Tree," hangdog Chris O'Dowd ("Bridesmaids," "The IT Crowd"), finding his life stalled after losing a girlfriend and a job in short order, goes in search of his roots and relatives. It's a trip that takes him into the theater, a boxing club, England's rural north, the back end of pantomime horse and finally to America. Michael McKean, a regular member of Guest's repertory company, plays Tom's father; Nina Conti his troubled ventriloquist sister. Jim Piddock, another Guest player, co-wrote the series and also appears in it, as Tom's antique-dealing downstairs neighbor. Familiar faces Fred Willard, Bob Balaban, Ed Begley Jr. and Amy Seimetz will also arrive in due time.