The Oscar-winning actress confessed that she was quite wary of De Niro when they first worked together many years ago.
Keaton told Vanity Fair magazine: "Mr Robert De Niro. My God. A legend and also a dear man. He's very kind. It's like, 'Oh, he's so kind. Why is he so kind?'
"And he's quiet. But he's a lovely man. I really like him. He loves his family. You don't think of him that way when you only know him from a distance.
"I did a movie with him called Marvin's Room a long time ago, but he was a producer. I didn't really know him and he was intimidating to me. But things change."
The 66-year-old actress also revealed what she loves to do in her spare time, saying that she enjoys walking around Los Angeles so she can listen to new music.
Keaton said: "The kind of music I listen to is when I'm walking, because I walk all over the neighbourhoods of Los Angeles. How else am I going to see it?
"If I don't drive, I take the dog and I listen to something. Right now I love the new Frank Ocean CD, Channel Orange. It's just amazing music.
"I also like the Jay-Z album with Kanye West. The last one - The Throne album. I just think that's so astonishingly beautiful, the way they orchestrate it.
"That's how I got to know who Frank Ocean was. Frank Ocean sang on one of the cuts. He [is] very orchestral in a weird way."