WASHINGTON — Legendary actor Bruce Willis’ brain “is failing him” but remains in “really great health overall” amid his dementia battle, according to his wife Emma Heming Willis.

“Bruce is still very mobile. Bruce is in really great health overall. You know, it’s just his brain that is failing him,” Heming Willis told ABC News’ Diane Sawyer in a preview that aired Tuesday morning of the special “Emma & Bruce Willis: The Unexpected Journey.” 

Heming Willis, 49, has now written a book about the little known disease Bruce Willis is facing and opened up about how subtle symptoms began when the film and TV star seemed subdued. 

“For someone who was very talkative and very engaged, he was just a little more quiet and when the family would get together, he would kind of just melt a little bit,” Heming Willis told Diane Sawyer. 

Eventually, Bruce was diagnosed with FTD – which is estimated to impact around 60,000 people in the U.S. It’s one of the most common forms of dementia for people under age 60. While there’s currently no cure or treatment for FTD, research into treatments is ongoing, according to the Mayo Clinic. 

As his condition has progressed in recent years, the action star’s “language is going” but there are still “moments” when the old Bruce Willis shines through, Heming Willis tearfully recounted. 

“It’s his laugh, right? Like, he has such a hearty laugh and you know sometimes you’ll see that twinkle in his eye or that smirk and you know, I just get like transported and it’s just hard to see because as quickly as those moments appear then it goes. That’s hard. But I’m grateful. I’m grateful that, you know, my husband is still very much here.”

As part of Tuesday morning’s preview, Diane Sawyer revealed she was able to spend some time with Bruce and will discuss that when the special airs on Tuesday evening. 

Emma’s book “The Unexpected Journey: Finding Strength, Hope, and Yourself on The Caregiving Path” is in stores Sept. 9.