The Rain Man star stepped behind the camera to tackle the story of a group of retired opera singers and he encouraged Smith to put more of her own life into her portrayal of a demanding showbiz veteran.
He also extensively researched the Downton Abbey actress and asked her to use some of her own quotes from past interviews in the film.
Hoffman told reporters at the American Film Institute Festival on Sunday (04Nov12), "We all know she's a diva. Her reputation is not dissimilar to my own...
"We were on the set and she suddenly said, 'I don't know what this scene is about.' So I sent the crew out, sat her down and we had a long talk.
"I believe all good work has to be autobiographical so I said, 'Maggie, this is not a character, this is you.' We're both in our 70s and I want to know what that feels like for you.
"I love Google, it's amazing and when using it to do research on her I'd found an interview she'd given talking about being ill, getting old and her view of the future. I got her to say some of those same quotes in the film."