Christopher Guest has worked on films including This Is Spinal Tap. His first TV show for the UK is Family Tree...

Family Tree is semi-improvised. What is it about this type of film-making that makes you keep on working in this style?

I have been doing this style of work for a very long time. I was involved in a film that Rob Reiner directed called This is Spinal Tap with Michael McKean and Harry Shearer and we wrote that story, which was an improvised movie. I remember having a lot of fun doing that and I thought, "Well, why not have fun". The spontaneity of it seems to help for the laughter element and comedy must have laughter. So we were hoping on this project to create situations where people would laugh. So that's why I have worked in this style for a while, hopefully to make people laugh.

Can you explain the premise of Family Tree please?

Family Tree is a story about a young man who has lost both his job and his girlfriend. Then he is left a box of curios by his great aunt in her will and the story really concerns his looking into his own family tree.

Could you explain the process of improvisation on the show?

It's difficult to explain how an improvised project works. All of my films have an incredibly strict story line. Every scene is mapped out and that is also true for this show. The dialogue is improvised, which is the way I usually work, and I think it results in very spontaneous scenes. Once we have the histories to the characters I give the actors some direction. It's like musicians getting together and playing. They know what the story is and we just start playing, and people know when to play and when not to play. That's the only way I know how to describe it really.

You prepared the series with Jim Piddock. How long did that take?

It took us a long time to plot out the back-stories to all the characters. We worked for probably seven or eight months on that before we were actually doing this for real. The character breakdowns went into each of their histories including where they went to school and where they grew up. It's quite detailed, so the actors had a very specific place to work from.

What was it about Chris O'Dowd that led you to cast him as Tom?

I was looking for an actor who was funny and who could also be sympathetic in an emotional sense, because it's more than just a comedy. I met Chris O'Dowd and I'd seen his work before, liked what he had done and in the end it was a great meeting. I just thought this is the guy.

There are two other important characters that are close to Tom Chadwick. What characteristics and qualities were you looking for when casting these key roles?

With work that is improvised, it's very important to get people who can do just that. Two of the other very important characters are Nina Conti, who plays Tom Chadwick's sister, Bea. Nina is a ventriloquist who has a monkey puppet. She is a wonderful actor and an amazing ventriloquist so the part was written for her.

Tom Bennett plays Chris O'Dowd's best friend, Pete Stupples and is the perfect contrast to Chris. Pete is not as smart as Tom but they are very close friends from childhood and Tom Bennett is an incredibly funny actor with a great ability for physical comedy.

You have worked with Michael McKean many times in your films. What's it like to write and perform with your friends?

Michael McKean is an old friend and collaborator of mine who plays Chris O'Dowd's dad Keith in Family Tree. Michael is an American playing a Brit and I really wanted someone that I could trust to be funny and improvise well - that was Michael McKean.

You consistently work with a great band of actors, many of whom are with you on Family Tree - how important is it to work with your regular repertory of actors?

I tend to work with the same actors a lot because they are the ones that can do this kind of work to a very high level. We were fortunate to have the best in the business including Ed Begley, who has been in several of my films, Bob Balaban, Kevin Pollak and Fred Willard who is revered in this world of improvised comedy.

How important is the layering of detail in Family Tree?

The costumes and props play an important part in this show, from the really dreadful sitcoms that Keith watches (which are specially written by Jim and I), to the actors who perform them. Equally the costumes played an important role because they had to range from the 1860s, 1880s, 1930s and 1970s to present day and that is a different challenge. Every actor had a huge amount of input into what they wore.

Tell us about some of the scenes you filmed in America...

Tom discovers he has some relatives in the US and he travels out to find them. Bea and Tom's best friend Pete are fed up with London - it's cold and miserable. They experience a lot of adventures with Tom's newly discovered relatives, some of which are very surreal but a lot of fun to film. Let's just say that they are as diverse and wacky in the US as they are on the English part of his journey.

The exploration of ancestry holds strong appeal for many people, perhaps more today than ever before. Why do you think this idea will make good television?

I don't think anyone ever knows what's going to be good television or bad television. I do what I do because it interests me and hope maybe other people will like it. I think the subject of genealogy is very big right now. People are drawn in because of the internet to various sites where you can go and find out a lot about your family in quite a short space of time. It's magnetic and you get drawn into it. I have done that myself - and it was part of the genesis of this show.