For the holidays, Greenwood stars in a touching made-for-TV movie, based on Greg Kincaid's best-selling novel, "A Dog Named Christmas." It's part of the "Hallmark Hall of Fame" series and airs on CBS Sunday night.
Greenwood tells CBS News he was so inspired by the theme of the movie that he wrote and recorded a song about a man and his "best friend" ashe was making the movie. Executives of Hallmark liked the song, "My Best Friend," so much that they decided to use it over the end credits of the movie.
Greenwood sat down with the Early Show to talk about more about his love for dogs and the unexpected impact "A Dog Named Christmas" had on him.
Initially, Greenwood admitted, he didn't want to get involved with the film, but when he read the script, he opened up his heart to the role as well as the cause.
"'You might really like it,' they said. So I said, 'OK. OK. Send it to me. I'm not even going to read it.' " But it was just such a gentle story. And quite often, I'm called upon to be such a b--tard, he joked. "You know, like in 'Double Jeopardy,' 'Rules of Engagement.' I've just played a lot of damaged guys, and it's nice to play someone with both feet on the ground."
In "A Dog Named Christmas," Greenwood, who grew up with "lots of mutts," portrays a father who initially doesn't want to adopt a dog for his special needs, adult son.
Greenwood said that, in some ways, he related to that as a parent.
"The rationale, as a parent, is to assume your child won't be able to care for the dog, and then they'll have to go through the heartbreak of giving it back -- especially when, nonetheless, you fall in love with it anyway," Greenwood said. "Basically, the son in this film won't be able to take responsibility for the pet no matter how much he loves it.
"And my character, the father, had two dogs, and even the one who lived for a long time left my character heartbroken when he died. And the character still feels that loss."
We hear a lot of stories about kids with learning/developmental disabilities who flourish when a pet is brought into a home. Was that the case with your son in the movie?
"This particular child takes in foundlings, birds with broken wings. He's pretty high-functioning, responsibility-wise, before his father consents to bring home the dog," he said.
As a dog lover, Greenwood admits, he fell in love with the dog who played Christmas in the film.
"How could you not? Some dogs are driven by love. This dog was incredibly, particularly food-driven. So, of course, I liked him! And we were affectionate, but the handlers really don't want you to get too affectionate. They want the dog to perform," Greenwood explained.
Does he think that hosting a pet for the holidays is a helpful step for families not sure if they want to adopt?
"It's difficult for families to decide what they want to do. If you can bring the dogs home -- that's the hope that you'll keep it. But if you decide not to, it is nice to have that unconditional love over the holidays," he said.
To find one of the 2,000 animal shelters near you participating in Petfinder's "Foster a Lonely Pet for the Holiday" program, click here.
To foster one of the pets seen on "The Early Show" on Thanksgiving Day, click here: