Diane Haithman is contributing to Deadline’s TCA coverage.

There are a lot of slashes in the titles of the guys in charge of HBO’s new series Family Tree, making its debut in the spring. Christopher Guest is writer/director/executive producer, and Jim Piddock is writer/creator/executive producer and also plays the role of Martin Pfister in the single-camera, documentary style show. (Executive producer Karen Murphy was not present).

Only actor Chris O’Dowd, who portrays Tom Chadwick, has no official title slash. But even his work calls for an extra level of creativity, since the show’s structure will reflect the improvisational mockumentary elements of Guest’s movies. “It’s very freeing, but very pressured at the moment,” said O’Dowd.

Related: Before ‘Downton Abbey’ There Was ‘Parade’s End’: TCA

Dowd’s character, Tom, is a 30-year-old man who has recently lost his job and girlfriend, and goes on a quest for his family identity when he inherits a mysterious box of memorabilia from a great aunt he never met. That story, Guest said, is based on his own experience going through the inherited belongings of his father, who died 16 years ago.

And today’s freewheeling TCA panel on the show reflected the process of creating comedy with an improvisational edge. Guest in particular enjoyed sparring with a questioner with a distinctive British accent. “Real voices, please,” he joked when the questioner first began to speak.

Related: Big Names Tackle Big Subject Matter In HBO’s ‘Phil Spector’: TCA

Guest was off on another riff when the questioner asked if Guest had found any dark family secrets among his father’s belongings. “Never mind that story about digging through a box of rubbish – your own roots,” the journalist demanded. “You have a few half-brothers you discovered, or already knew about?”

No, no, I know who my family are, actually,” Guest said mildly, sounding both bemused and amused. “I have a half-brother, a brother and sister. I knew them. Actually Mom and Dad had met.” He did go on to answer the question more seriously, saying that his search through his own family memorabilia, some of it several hundred years old, served as inspiration for “interesting funny turns that make this amusing, I think.”

Related: HBO On ‘Mea Maxima Culpa: Silence In The House Of God’: TCA

Guest said that he and Piddock had considered making their idea into a movie but that the continuing nature of family research lent itself better to a television series. He said that four episodes of the show have already been shot in the UK. The show, a co-production of HBO and BBC Two, will make its debut in the spring.

For all of Deadline's headlines, follow us @Deadline on Twitter.