"The two of them had been looking for each other for years, without each other knowing," he said.
Coogan's character, Martin Sixsmith, joins Dench, who plays Philomena, on a journey to find her long lost child. "In some ways," he said, "it's a road movie -- a buddy movie, an odd couple."
Without giving away the ending, Coogan, who was joined by director Stephen Frears on "CBS This Morning", said it has a tragic ending, but added, "There's a lot of humor, a lot of comedy in the film, which I think people don't expect."
Coogan said he pitched the movie to Judi Dench, while sitting in her kitchen. He had to tell her the story, because her eyesight is not good these days. "She has great trouble with her eyesight," he said. But after hearing the story, "she said I want to do it."
The movie, "Philomena", has grossed more than $46 million in sales worldwide, after generating only $128,000 its opening weekend. It is nominated for three Golden Globes -- for best motion picture drama, best actress and best screenplay.