The LORD OF THE RINGS star was struggling with his accent for the film and took weekends out before shooting started to visit the city and chat to locals.
And when THE COOLER star Bello found out, she put him in touch with her own Philadelphia relatives who have thick accents.
Mortensen says, "Maria is from Philly and her family is from south Philly. I went down there a few times on my own to do research and she found out the last time I was going there and said, 'You ought to call my Uncle Pete or my brother.'
"I did get together with them and they were very helpful. They helped me see that world a little bit better."