How my son's disability is helping me to direct Joe Egg

Mostly I howl with laughter, but there are also bitter tears, says Stephen Unwin

Helping hand: Jessica Bastick-Vines as Joe and Ralf Little as Bri in 'A Day in the Death of Joe Egg' (Sim Annand)

The more plays I direct, the more I know that I don't direct them well unless I bring my own experiences into the rehearsal room.

But nothing I've ever worked on has been as close to me as Peter Nichols' remarkable black comedy about the frustration and desperation of being the parent of a child with a serious disability. My 16-year-old son, Joey, is very different from the 11-year-old Josephine with severe cerebral palsy who is at the heart of A Day in the Death of Joe Egg, but there are many things – beyond the name – that I find achingly familiar.

Joey suffers from profound and multiple learning disabilities. Like Josephine he has severe epilepsy, no speech at all, wears a nappy at night and is very vulnerable, especially when he's ill. But he's mobile, and, for the most part, physically able. The real parallels lie in the everyday impact that such children have on those around them, especially their parents: they can never be left unattended, the medication has to be right and dealing with their physical needs is grindingly hard work.

But there's the dark psychological stuff too. Quite properly, parents of children with disabilities are encouraged to look at the positives – the glass is "half full" – but that doesn't dispel the disappointment and grief that even the strongest succumb to – and which so often drives parents apart. Jokes are common but so also are unformulated feelings of guilt. It's a bitter pill to accept that the condition is permanent and I often feel an emptiness in my stomach watching Joey's bus go off to special school, full of children whose lives will always be so different. "Why me?" we ask. Well, "Why not me?" is the only answer.

There's also the way that the child is seen by others: the relatives who are eager to help but have their own busy lives; the well-meaning friends who imagine there must be a solution but have little understanding of the gravity of the situation; and the doctors, educationalists and specialists who do so much but can't answer any of the big questions. And all the time, we see our disabled children reflected in other people's eyes: mostly as objects of sympathy and care but sometimes of disgust or fear. One of the characters in the play articulates the majority view when she describes "poor Joe" as a "weirdie".

Because Peter Nichols is such an honest writer (he had a daughter with cerebral palsy who died at the age of 11) he gives legitimate voice to the darkness that many parents spend so much energy trying to overcome. Sharing these feelings is sometimes painful and I know that I must avoid making this into "A day in the life of Joey Unwin".

But there's a moment in the play I find particularly hard. It's when little Joe comes skipping on to the stage and announces in a bright voice that there will "now be an interval". This is her mother's fantasy about her daughter as just another 11-year-old. I have a similar dream of Joey being able to talk – I can even hear his voice. The fact that such children will never announce the interval and that no miracle cures are possible is the hard truth that Peter Nichols anatomises with such brilliant effectiveness.

Most of the time, rehearsing Joe Egg makes me howl with laughter but sometimes it draws bitter tears of recognition. The actors generously say that my rollercoaster of emotions is useful; I hope that my own experiences will give the production the stamp of truth.

'A Day in the Death of Joe Egg', Liverpool Playhouse (everyman playhouse.com) Friday to 27 April; Rose Theatre Kingston (rosetheatre kingston.org) 30 April to 18 May