Francesca Simon, writer

Orion, my publisher, wanted a book for very young readers, so I wrote a stand-alone story, Horrid Henry's Perfect Day, about two brothers, one horrid and one perfect. The editor rejected it as too challenging for the age group, but was keen to rework it and asked for four more stories to publish with it. I was worried: I thought I didn't have any more stories in me, but they flowed easily. Henry was an embodiment of that impulse in all of us – to rule the world and get our own way. Rather than modelling him on a specific boy, I used aspects of myself: the aggressive, selfish, get-out-of-my-way instincts you see surfacing in adults. As the eldest of four siblings who grew up in a cramped house, I was familiar with Henry's longing for freedom. Although I was a model pupil at school, I was – like Henry – loud and disruptive at home.

Perfect Peter was the counterfoil, the part of us that tries to be good. Since he was meant to be the child we all try to socialise our own children into being, I was surprised when I found that many parents disliked him. His jealous relationship with Henry is the original Cain and Abel battle seen in most households: if one child is good at something, the other goes the other way to be different. My niece and nephew were always fighting: when one got A grades, the other started getting Fs.

Tony Ross had free rein to draw the characters exactly as he saw them. It wasn't until Horrid Henry's Nits was published, three years on, that the books started to attract attention. Nowadays, if authors haven't got noticed after two books, they are cast adrift. So had I begun it 10 years later, Horrid Henry would never have had a chance. For a while, I was worried about being tied down by their success, but I have deliberately limited myself to writing one Henry book a year. And I love doing them. They make me laugh.

Henry gives the illusion of being a monster, but he says and does nothing that other children don't do. If he were to plot and scheme, he would be a borderline psychopath, but in the main he reacts to situations to get his own way, which makes him funny and endearing rather than alarming.

I feel reading about his behaviour gives children the thrill of being bad – without the consequences. Henry says things that we all think but don't dare express, and I'm pleased when parents and children read the books together. It sends kids the message that it's normal to sometimes hate your sibling or parents. We often try to override how children feel – telling them they're not really scared, or that of course they don't hate their brother.

Tony Ross, illustrator

I didn't instantly visualise the characters when I first read the books – it was more a case of simply trying to differentiate between them. Henry had to be recognisable from every angle, so I gave him a stripe round his jumper and long hair, which I've always associated with rebelliousness. One of my first primary school teachers floated into my mind when I came to Miss Battle-Axe, Henry's teacher, who is the character I enjoy drawing most. I like bad people with waving fists and scowls, so Henry is a joy, too. Peter was more of a challenge, because an innocent posture can look bland, so I tend to put his hands behind his back and turn his face up. Henry's mother was the most elusive character. I realised I was drawing mums like they used to be, so I gave her jeans and trainers to make her trendier.

It takes me a week to illustrate a volume of four stories, working flat out – or my version of it, which involves long lunches and finishing in time for Coronation Street. I'm now so familiar with the characters that they almost draw themselves. I did make one terrible mistake that no one has yet noticed: in Horrid Henry's Evil Enemies I drew the jersey yellow with a blue stripe when it should be the other way around, but even the publisher didn't pick up on it.

Sometimes it rankles that I am known now for Horrid Henry when I've illustrated 2,000 other books, and done many of them much better. On the other hand, the success of the series has been a pleasant ego trip. It's satisfying at signings to see a queue of people stretching into the distance, all clutching their copy.

• Horrid Henry's Nightmare by Francesca Simon and Tony Ross, is out now, published by Orion, price £4.99