The Rachel Getting Married star begged director Howard for a part in the 2005 biopic and she's convinced she only got an audition because of her links to the gritty boxer Russell Crowe portrayed in the movie.
She tells WENN, "I felt so blessed to be part of that. Ron Howard gave me a courtesy read because he knew the familial connection.
"After I read he saw I could act but I'm sure I was the sentimental favourite.
"Russell was amazing. My grandfather died when I was three, so it was nice because while they were trying to flesh out the script they kept asking me for personal references... and my dad didn't even know he had a box of love letters from Jim to (wife) May that we would have probably never known we had in the attic unless someone was doing research.
"It ended up opening a whole new window into my grandparents."