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John Thomson shot to fame with lead roles opposite Steve Coogan and in The Fast Show and became a household name when he was cast as Pete Gifford in Cold Feet.

A husband-and-wife team, the clowns' place in the circus is wholly down to their family connections, and is nothing to do with any talent.

Geoff, played by John, is the brother of Lizzie's dad, the previous ringmaster and owner of the circus, who is now in prison. Lizzie would dearly love to sack her uncle and aunt, but just hasn't got the heart.

On stage his wife Helen, played by Sophie Thompson, functions as a sort of straight man to her husband, who she absolutely adores and considers to be a comedy genius... which he isn't.

Geoff is a dedicated clown, but doesn't have a funny bone in his body. He has read every book on the theory and history of comedy (he's even written one), but when it comes to actually making other human beings laugh, he just can't.

This doesn't dent his confidence in any way. If the audience aren't enjoying themselves, it's their own fault for not concentrating. And he'll keep entertaining them with his bizarrely dark acts until they laugh. And if they still don't laugh, he gets angry.

Deep down Geoff probably knows that he is a lousy performer, but he covers it by having a large ego. He would love to be a stand-up comedian, and writes off for auditions, but is constantly rejected. Geoff hates Boyco because he is talented and in love with Lizzie. He hates the idea of them ever getting married – it would put Boyco in charge of the circus, and he is already the star of the show. Geoff therefore undermines him at every opportunity.

But there is another side to him when he's alone in the caravan with Helen. He's much more vulnerable, and she's the powerful one. He needs her to bolster his confidence, which she duly does – and he does what she tells him to.

John takes up the story: "Geoff has issues with anger management and with kids! Children's laughter goes right through him, which sums him up really!"

He continues: "Sophie and I play Helen and Geoff, but we're also the Thompsons in real life, both with and without a 'p'. We're the clowns, Plinky and Plonky, but we didn't go to clown school. I'm a rubbish clown – but it's a bit like with Les Dawson. You've got to be a really good piano player to play badly and that's what I'm sticking to! I'm such a bad clown because I have total faith in myself to the point of ego-mania, which destroys any element of entertainment.

"In the circus ring it would appear to the crowd that Geoff wears the trousers. But behind closed caravan doors Helen is very much the boss and Geoff is like a child! She keeps him in check. Helen can also interpret the worst review in the most positive way. We love each other dearly, but Geoff is very difficult as he has a lot of issues."

John and Sophie get to wear some wonderful, colourful and ludicrous get-ups: "Make-up takes a while for us. I did quite a bit of research into clowns and I was a bit worried about stealing somebody's face as they are all sort of patented! We are August Clowns, which are the knockabout clowns – not the ones with the white faces and the pointy hats or character clowns like Charlie Chaplin."

August (or Auguste) Clowns became popular during the second half of the 19th century, They wear colourful, ill-fitting clothing, often mismatched and oversized, have bulbous red noses and brightly-coloured wigs. Their shoes are often oversized and exaggerated and, from head to toe, colour is all important.

John continues: "I think that clowns have a rotten time of it. People used not to be scared of them, but a lot of clowns have ended up in horror films, like Stephen King's IT, which hasn't helped, and there was a serial killer who dressed as a children's entertainer.

"A lot of clowns just aren't funny these days – like Krusty the Clown from the Simpsons. Children read expression – they know when people are happy, cross with them, sad and so on. But with a clown the expression is a permanent thing and so the child gets confused because they can't read what's going on behind the make-up. It's more a primitive fear of clowns rather than what the media has generated. It's all to do with trust – and they can't read a clown's face."

He adds: "There's actually a word for it – fear of clowns is claurophobia!"

In fact Johnny Depp and Sean 'Puff Diddy' Coombs both admit to suffering from it. Nonetheless, John himself is a big fan of circuses: "I used to go to the circus at the Blackpool Tower all the time when I was a kid, because I grew up in Preston which wasn't far away.

"When you're little everything seems so big, but, actually, it's all really small. It's still packed out today. They still flood it and there's still a water show at the end. I loved the circus when I was a kid!"

John also loved the authenticity of the Big Top set: "Plinky and Plonky live in a proper circus caravan, with the original carpet and clock. A classic 1970s caravan. It's even got its own porch!"

He adds: "I haven't done a sitcom since Men Behaving Badly and I had forgotten just how full-on it is filming in front of a studio audience! I wanted to do my own stunts, but the insurers didn't. When my hat and shoes catch fire, for example, the stunt man did that!"

He laughs: "No custard pies were thrown when we were making Big Top simply because of the continuity – if you throw one or rub someone's face in it all the make-up will come off and if we got the lines wrong we'd have to go back into make-up and everything would end up taking so much longer."

Geoff's wife Helen is played by the versatile and award-winning actress Sophie Thompson who is probably best known for playing Stella Crawford in EastEnders.

Helen is a mother figure to Geoff and sometimes the rest of the troupe, especially Lizzie. She has a heart of gold. On the surface she doesn't appear quite on the same planet as the rest of us, but constantly surprises with comments that are often reason in madness. She is very honest and often misinterprets the threads of conversations, which makes her appear naive. Sarcasm, for example, is one of her blind spots.

Helen loves Geoff unconditionally, but knows deep down that he is rubbish. She's like a proud mum and protects him, where she can, from the world, constantly boosting his fragile ego. She even rewrites his rejection letters when he applies for stand-up comedy jobs.

With another partner, Helen might actually have been quite a good clown, and she has lots of ideas for the act, but is never allowed to use them. Geoff is in complete control.

Helen has learned to accept her fate – she's resigned to her marriage and ever-optimistic about life, but has low expectations.

Sophie takes up the story: "Helen's confidence has been rather knocked by Geoff, who never allows her to perform to her full capacity because of his ego, and she really starts to come out of herself when she's not performing with him.

"She gets a real chance to shine when she performs with Lizzie in one of the episodes. People actually laugh at the clowns when Helen is allowed to perform without being in Geoff's shadow."

She continues: "Big Top is very much like Noises Off, where you don't get to see a lot of the actual circus performances (we'd all be rubbish!) you see more of the backstage world and the behind-the-scenes goings on. Which is far more interesting!"

Sophie got to wear some very colourful outfits – including colourful voluminous frocks and enormous, outsize shoes, which were not as impossible to walk around in as they look: "We have slippers inside our clown shoes to make them more comfortable and fit better."

She is married to the actor Richard Lumsden and they have two small boys: "My family and I have been to the circus quite a lot. In fact quite recently I took my children to see the Chinese State Circus."