Michael McKean, who played David St. Hubbins in This Is Spinal Tap plays Keith in Family Tree...

Tell us about Keith. What kind of man is he, what is his situation in life?

Keith has always been in service and was twenty years in the army before becoming a Beefeater at the Tower of London. He had his eye on the Master of the Raven gig but politics got in the way. So he decided to blow that off after twenty years and became an inventor. In a sense he's evolving too - cutting himself off from the past, and starting a new chapter in his life in his sixties.

He has got his lovely wife, played by the amazing Lisa Palfrey, who is one of my favourite actresses and just an awesome person, funny as hell. And boy does she do her homework. Oh my lord! Oh she learned a whole bunch of Moldovan and scolded us about our table manners. She said "In Moldova, you don't put your napkin in your lap, you keep it on the table and if someone pours you a drink, you can't put it down until it's finished".

What's the relationship between father and son?

Keith cautions Tom about looking too deeply into the past because he just doesn't see the point. He doesn't really understand this quest but he also respects Tom's judgement. He's kind of worried that Tom will become a hermit now that he has been dumped by his girlfriend.

I am positive that maybe Keith lived a celibate lifestyle for quite a while, between the very difficult wife who's long, long gone and Luba. It's kind of a mystery but also something he looks back on with some pain perhaps and he knows that a lonely life is only good for so long and it will only take you so far and he's very happy right now I think. He's happy and he and Luba found each other.

What's his relationship like with Luba?

What's good about his relationship with Luba is that it isn't about a gold digging mail order bride. They are just crazy about each other and it's nice. It really works for both of them. Their journey is a two person journey that took them long enough to find but I think that is exactly who they are. Couples like that usually die within a year of each other so I hope Luba is taking care of her health!

Have you ever looked into your own family history?

One strand of my people came from Ireland and went to the United States before it was the United States in 1723. One of my ancestors, Thomas McKean signed the Declaration of Independence, a document of some note. Our family crest said "I'll get around to it" and he did eventually! So I know certain little things like that.

There was another relative who actually hung out with Davy Crockett and boy, when I was 10 years old that blew the top of my head off - I said "Man, it doesn't get any better than that." I still had to go to school and get pushed around like everybody else, so it doesn't really buy you much. I think that if you happen to dig up the fact that your great, great grandfather was a serial killer that is going to give you some digestive problems, but it's really not going to change who you are

What is Keith's obsession with bad 70's sitcoms?

Wow. You know, I think it's Keith's comfort food. For people who have grown up in the last 25 years in America, their comfort food is Law And Order. A generation before that they had I Love Lucy. There are certain things that just hit the spot and take you to a time where everything kind of made sense. Sergeant Bilko is another one that stays funny, it stays good. Laurel & Hardy will never be not funny. I mean those things. I think in Keith's case the stuff that he is really into is pretty dire but you see what it is about. It's about returning to a time when things were simpler.

Does Keith think he will become a successful inventor?

I think if he ever gets an idea that makes any money at all, it will probably be some dumb little novelty like a singing flower-pot. What Keith lacks is the science. He had an idea about paint, that as long as it is wet it makes a sound so you don't actually walk into it and touch it in the dark and then when it dries it stops making the sound. Now, he doesn't have the science for it but he's got the dream, you know what I mean?

Where do you start with a character like Keith? Do you have your own process now for working with Chris?

I play by the rules as much as I can. Apart from the inventions, I don't think Keith is primarily a comical character. I think he's kind of stolid in a way, which is not particularly challenging but it's kind of fun, and nice, you know. I've played the other types of characters too - the mad men. I've been working on stage a lot which is great because, you really never know what's going to happen, what the audience is going to throw you or what another actor might forget so I try to stay on my toes in that sense. When you are working on a stage you have somebody else's words in your head but with Chris Guest I guess we are all on a tightrope.

What are the kind of conversations you have as a group of actors when you are about to approach a scene where everything is fabricated?

Nina Conti had done a few things before with Chris, such as For Your Consideration, she also did a little Spinal Tap film that we did - a promotional film for this album we had out a few years ago. But Chris O'Dowd was a little more inquisitive. He wanted to know what the process was and I said, "I just try to make the other guy laugh and then he tries to make me laugh and we stay on story". Of course the minute the camera started to roll it was like he'd been doing it all his life.

You see, that's what doesn't change, whether you have the lines written for you or whether you are making it up as you go, your intent, your goal, your action, that's what you have to know and as long as you are aware of that you are going to be OK. You might not come up with the punchline of the century - that's Jennifer Coolidge's job but, you will be on track and the scene will happen.

You have been personally responsible for some of the most iconic characters in improvised comedy. Is there a moment when you are playing with a new character, when you are in a scene when you feel like it is working and you have created something authentic?

The best moment is when you don't think about anything except what is going on in the scene. That's when the magic really happens.

What did you enjoy about the London leg of the shoot?

I had an amazing time in London - I love this town. The only problem was that my wife wasn't with me. It can be a lonely town if you don't know a lot of people but I do a lot of walking. London is the best city for walking and I saw a little bit of theatre too while I was there.