John plays Ashley Thomas in the ITV serial and the one-off instalment will focus on his character's struggle with dementia.
Emmerdale bosses have worked closely with Alzheimer's Society and MHA (Methodist Homes) to produce the episode and it will give viewers a real insight into living with the disease on a day-to-day basis.
Although the exact content is being kept tightly under wraps, Emmerdale series producer Iain McLeod says: "People living with dementia face challenges most of us can barely imagine. So, I took it as a challenge to help people picture this experience - to put them inside the mind of someone living with this condition.
"With this chapter of Ashley's story, we set out to give people an insight into how ordinary, day-to-day experiences can become disorientating and distressing when refracted through the lens of dementia. Catching a bus, the apparently simple act of buying something in a shop, holding a conversation - all of these become tasks of Herculean scale.
"By telling the episode solely from Ashley's point of view and seeing things the way he sees them, I hope we're showing a side of dementia that is seldom represented on television."
Last year, vicar Ashley was diagnosed with stroke-related early onset vascular dementia and his condition has steadily worsened. The special episode will focus on Ashley leaving hospital and heading out into the streets alone.
Iain adds: "At its core, this has always been a love story between Ashley and his wife Laurel, and this episode is a heart-breaking landmark in the evolution of their relationship."
Kathryn Smith, director of operations at Alzheimer's Society, praised the work of the soap, saying: "We are incredibly moved that Emmerdale will be filming an episode dedicated to showing the audience the world from Ashley's point of view.
"Storylines like this can help people raise awareness of dementia and help them to understand how dementia can impact a person's ability to think, communicate, process new information and break down the stigma that is still associated with the disease."
Theresa Knight, from MHA, adds: "We have been closely involved with Emmerdale since the team contacted us wanting John to visit one of our care homes for people living with dementia. Since then we have been involved in feeding back on scripts and storylines involving Ashley as his condition deteriorates.
"We have been delighted as to how seriously the whole team at Emmerdale is taking the storyline and the lengths they are going to in their research."
The special episode will air next month.
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