The actors previously shared the screen in 'The Expendables.'
"I think that he's fantastic," Schwarzenegger told Cinema Blend of his co-star in The Expendables. "He was, always for years and years, is a huge, huge star. And I think that he will always be remembered as a great, great star. And a kind man. I understand that under his circumstances, health-wise, that he had to retire. But in general, you know, we never really retire. Action heroes, they reload."
Willis, 68, stepped away from the spotlight last year amid a heartbreaking battle with aphasia, which his family said at the time was "impacting his cognitive abilities."
In February, the Die Hard actor's family shared that his condition has progressed from aphasia to frontotemporal dementia, also known as FTD. In a joint statement (via the Association for Frontotemporal Degeneration organization) they called it "a cruel disease."
"As a family, we wanted to take this opportunity to thank you all for the outpouring of love and compassion for Bruce over the past ten months," the statement read. "Your generosity of spirit has been overwhelming, and we are tremendously grateful for it."
According to the Alzheimer's Association, FTD "refers to a group of disorders caused by progressive nerve cell loss in the brain's frontal lobes (the areas behind your forehead) or its temporal lobes (the regions behind your ears). The association also says nerve cell damage caused by FTD can lead to "loss of function in these brain regions, which variably cause deterioration in behavior, personality and/or difficulty with producing or comprehending language."
At the time of Willis' aphasia diagnosis and retirement last year, a source told ET that he had been "struggling for quite some time."
"He has tried to keep his health struggles private, but last year, his health started declining more. He made a plan to make a bunch of movies and do as much as he could career wise, so that he could retire and take care of his family," the source said.
Last week, Willis' wife, Emma Heming Willis, took to Instagram to share a beautiful story about how the couple's 9-year-old daughter, Evelyn, is offering her support amid her dad's health struggle.
Emma, who also shares 11-year-old daughter Mabel Willis with the Pulp Fiction star, relayed that Evelyn had approached her to ask if she was aware that people with dementia "can become severely dehydrated." When asked how she knew about that issue, Evelyn said that she had used her free time in school to research "fun facts about dementia."
"Now, that's not funny, but it's kind of funny," Emma noted. "And she really is her father's child because these two love some random facts. So I said to her, 'Evelyn, we will always make sure daddy has a bottle of water in hand. Thank you for letting me know. But that is the most loving and compassionate thing that you can do is to be curious and educate yourself on your dad's disease.'"
In addition to Evelyn and Mabel, the actor is also dad to three adult children -- Rumer Willis, 34, Scout Willis, 31, and Tallulah Willis, 29 -- with ex-wife, Demi Moore. He also recently became a first-time grandfather, as daughter Rumer gave birth to her first child last month.