When Emma Heming Willis sensed something was off with Bruce Willis, she first thought it was his hearing loss from "Die Hard."

The former model has written a new book, "The Unexpected Journey: Finding Strength, Hope, and Yourself on the Caregiving Path." It’s centered on her personal experience as a caregiver for her husband, who was diagnosed with frontotemporal dementia (FTD). It also serves as a roadmap for new caregivers.

"When Bruce was shooting the first ‘Die Hard,’ he shot a gun underneath a table multiple times, and they didn’t have him wear any kind of ear protection," the 47-year-old told Fox News Digital. "That took out a huge percentage of his hearing. And over time, for all of us as we age, our hearing starts to go a bit."

BRUCE WILLIS SHOWED 'ALARMING' WARNING SIGNS BEFORE DEMENTIA DIAGNOSIS, WIFE SAYS

"That’s what I thought was happening," she reflected. "Maybe we’re having this miscommunication. He is really not hearing me properly. I thought maybe that’s why we weren’t communicating as well as we used to, because of his hearing."

In the book, Heming Willis wrote that after filming "Die Hard" in 1988, Willis "lost a large percentage of his hearing in one ear." It "never posed a real problem" when the couple first got together before marrying in 2009. 

Still, Heming Willis noticed that her spouse started to "check out" at dinners. She wondered if he was just letting his children, including his three older daughters, "yak it up," as he would say, and have "girl talk."

"I assumed his hearing loss made it easier for him to melt into his seat with his hands clasped gently on his lap," Heming Willis wrote.

Still, it wasn’t the man she knew. Willis loved spending time with his daughters and would often rush home to be with his girls. And there were other signs that something was wrong.

"Bruce had a severe stutter as a young child," Heming Willis told Fox News Digital.

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"When he went to college, he figured out how to work with that stutter. He’s always been a person that stuttered, but he managed it throughout his life. And for me, some of the first signs were noticing that his stutter was coming back. And … our relationship just felt very different. It all felt very different. Our conversations and our connection just felt off. It was really hard for me to understand why or what was happening to be able to put my finger on it."

"I never realized, or would’ve ever thought, or have known, that this sort of lack of empathy was a symptom of young dementia," she added.

On the surface, the now-70-year-old appeared healthy. He didn’t "flag" any changes he was experiencing, she said. And doctors didn’t spot any health concerns. Still, Heming Willis knew something wasn’t right. His communication just wasn’t the same.

WATCH: BRUCE WILLIS DIAGNOSED WITH FRONTOTEMPORAL DEMENTIA

"I just knew in my gut that something else was going on," she said. "That’s when I approached his doctor. And I think that’s so important for other caregivers as well. We know our person better than anybody. It’s important for us that we know it’s OK, and we should feel it’s empowering to go to our loved one’s doctor and say, ‘Something is not right, and I really need you to listen to me. Hear me.’"

In 2022, Willis’ family announced that the star was diagnosed with aphasia, a condition that causes loss of the ability to understand or express speech. Nearly a year later, the star's family said Willis had a more specific diagnosis of FTD. 

The Association for Frontotemporal Degeneration describes FTD as a group of brain disorders caused by degeneration of the frontal and/or temporal lobes of the brain that affect behavior, language and movement, according to The Associated Press. Aphasia can be a symptom of it.

The association describes frontotemporal degeneration as "an inevitable decline in functioning," with an average life expectancy of seven to 13 years after the onset of symptoms. The progressive disease is terminal and there’s no cure.

As Heming Willis was "free-falling" from the devastating news, all she was given was a "flimsy pamphlet."

"When I walked out of there, I'd never felt so alone, so isolated," she said. "I didn’t know another family that was going through this, especially with two young children. … I had to take the time to be able to search the World Wide Web to come up with some kind of plan."

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Over the years, Heming Willis has sought the guidance of doctors and experts to get a better understanding of FTD, its progression and what caregivers can do to give loved ones the best quality of life possible. She's been sharing her findings and conversations on social media.

"I love and care for my husband so much," said Heming Willis. "I want him to go into this next chapter of his life cared for and loved. But I think what has also helped me is the advocacy part of all this, being able to use my voice to help somebody else. That has been really comforting to me, to be able to share what I’ve learned along the way. I think being able to give back in any way I can has been the way for me to continue walking through this journey."

Through the pain of watching the disease progress, there are also many moments of joy, she said.

"It’s about being with my husband, just being able to sit with him," she said. 

"He is so present today. He is not thinking about yesterday or what’s going to happen later. He’s just in his body here. And for me to be able to just sit with him, hold his hand, hug him — and we laugh. We have joy. … Our lives are not over. Even with the diagnosis, the person is still human. There is still so much joy and so much laughter that we share with Bruce. We’re just in a different phase, a different chapter of our lives with him."

Heming Willis hopes that by going public with a private, devastating experience, other caregivers won’t feel alone in their journeys. But today, it's all about taking a day at a time, for herself and her husband.

"I’ve come a long way," she said. "I never thought since receiving Bruce’s diagnosis that I would find laughter again. I think it’s taken time. I'm still learning. But I’m doing the best that I can."