The "Lord of the Rings" actor, 62, directs and stars in Falling, which follows a gay man who looks after his sick father, Willis, in old age.
It's an incredibly personal project for the star as his mother, Grace, died in 2015 after battling dementia, while his dad, Viggo, Sr., passed away two years later after a similar fight.
"I've had experience with dementia up close," Mortensen tells British newspaper The Times. "I've had it with both my parents, and my grandparents, and my stepfather. And with my parents, it was in a care-giving role, where it was daily, hourly, by the minute."
Describing what drew him to make the movie, Viggo says cinema has largely misrepresented the topic and wants to change how people see dementia sufferers.
"Even in the best movies about dementia we're looking at someone who's confused," he explains. "And in my experience those who are confused are the observers. Generally speaking, the person who has dementia is not confused unless you correct them."
"Which is almost always a mistake. Don't tell them that the person they're talking about happily actually died 20 years ago! For what? They die again for them!"
"And if they are still in that in-between stage, where they can come in and out of it, they realise that they've said something stupid and feel doubly bad. You have to examine why you have the need to correct them. It's for yourself, because you want that person to be what they were. I wanted the film to show the present from the point of view of Willis. And I think we achieved that."
"Falling" is in cinemas from 4 December (20).