"It's just the most amazing story about the most extraordinary woman," said Dench.
A writer helps the woman embark on the journey. Dench says she was honored to be able to meet and spend time with the woman who inspired the fact-based film.
"It hasn't happened to me very often; it's only happened to me twice, I think. And of course, it makes you feel more responsible," said Dench.
The first of her 6 Oscar nominations came in 1997 for playing Queen Victoria in "Mrs. Brown." It gave Dench her first taste of dealing with the challenge of playing a real person.
"The night before we started filming, Mrs. Brown said to me, 'You know Queen Victoria was left-handed.' I hoped they were right. In this case, you had to be very sure that you were right because you had the person virtually standing beside you," said Dench.
Initially, the real Philomena was reticent to have her story told. But director Stephen Frears, as well as co-writer and co-star Steve Coogan, assured her the story would not be overblown or overly sentimental.
"Our responsibility very much to her was to tell the story truthfully and not exaggerate it, or make it more glamorous, or more depressing, but be true to her," said Dench.
Dench is a few weeks away from turning 79, and this theater-trained actress is as surprised as anyone at her successful transition from stage to screen.
"I never thought I would ever make any films, but I love it now having watched a lot of people who are very, very, very good at it and worked with directors who are very good at it. And the thing is once it's there, it's there -- that film will never change," said Dench.
"Philomena" was originally given an R rating for language. Producers, with some help from Dench, appealed that rating and got it changed to a PG-13. Dench was pleased, saying she was happy to now be able to take her teenage grandson to see it.