Coronation Street star Jane Danson, who plays Leanne Battersby, explained the tough road ahead for Leanne and Steve's son Oliver on This Morning.
He is set to be diagnosed with a mitochondrial disorder which is life-limiting, after suffering from seizures.
Speaking to hosts Holly Willoughby and Phillip Schofield, Jane ed, which is common in young children.
Leanne initially blames Steve for this, but then the seizures become more "frequent and concerning".
Steve and Leanne are at war with each other because they both have different ideas of what is wrong with their son and how to deal with it, she said.
But eventually it brings the couple even closer together as they get the "devastating" diagnosis.
The early signs something was wrong were in February, when Oliver was napping a lot.
"He goes to hospital and the medics are quite concerned as to why he's having these fits, and they can't stop it," said Jane.
"So obviously that rings alarm bells, and that's when the story takes a turn for the worst and in the coming weeks we eventually get the diagnosis for what's wrong with him and it becomes devastating for everyone involved."
Jane said it was "upsetting" to act out this storyline, during which the cast and crew have been seeking advice from The Lily Foundation, a charity to help raise awareness and support families affected by mitochondrial disorders.
Jane also revealed a surprising fact about the actors playing Oliver.
While it was already public knowledge that twin actors were used to play him, she also said that a third and older child is used for wide shots and stunts.
This is likely because there is a strict limit on how many hours a day child actors are allowed to work.
One viewer tweeted: "Watching Jane Danson’s interview, you can tell by her emotions about how much she has dedicated herself to her new storyline.
"This dedication as well as her talent makes me believe she will give this storyline its justice Leanne was definitely the right character to pick".
Another added: "As a mum that has lost 2 children to mitochondrial disease I thank you for your storyline".
A third tweeted Jane saying: "Just seen your interview. Great to learn more about this condition and looking forward to see it play out on screen!!"
A fourth penned: "Interesting interview. We heard how twins are used for Oliver but there's also a third child who is used for wide shots and stunts! Jane said she is proud of them."
* This Morning airs on ITV on weekdays at 10am
