Diane Keaton’s health had taken a sharp turn in recent months.

“She declined very suddenly, which was heartbreaking for everyone who loved her,” a friend of the Oscar winner — who died on Oct. 11 at age 79 in California — tells PEOPLE exclusively. “It was so unexpected, especially for someone with such strength and spirit.”

The source adds, “In her final months, she was surrounded only by her closest family, who chose to keep things very private. Even longtime friends weren’t fully aware of what was happening.”

Keaton had made at least one major lifestyle change in recent months: In March, she listed her beloved “dream home,” surprising many after she said she had planned to remain there permanently.

The longtime home-flipper, who was renowned in interior design circles for her sophisticated taste, had even written a book about the home, called The House that Pinterest Built. After moving there in 2017, she completed an extensive eight-year renovation. But in March, she listed the five-bedroom, seven-bathroom property for $29 million.

In her book, she noted that the inspiration for buying the home was the children’s fable The Three Little Pigs. When she was young and her mother read her the story, she took the lesson to heart. The third pig’s indestructable home, of course, “was made of bricks,” she wrote. “I knew I was going to live in a brick house when I grew up.”

In a 2017 interview with Wine Spectator, she said she always had “an interest in homes” but that she consistently struggled to “land and stay” because she found something wrong with every property.

Not so for her final L.A. house, however. “Something’s right, because I love it,” she told the outlet, describing the property as her “dream home.”

That wasn’t the only lifestyle change for the Oscar winner. Once a fixture in her neighborhood, where she could be seen walking her dog daily, she ceased to be seen outside.

“She lived in Brentwood for many years,” a source tells PEOPLE. “She loved her neighborhood. Up until just a few months ago, she’d walk her dog every day. She was usually dressed the same, with a hat and her signature sunglasses regardless of the weather.”

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The source continues, “She was always very nice, funny and chatty. She’d talk to her dog like he was a person. She was eccentric and had this old-school Hollywood aura. She was very, very special.”