But the story takes a dramatic twist when Willis, a police officer, realizes the prisoner is about to testify against other officers, including his ex-partner, played by David Morse.
Both Willis and Morse visited The Early Show Wednesday morning to talk about their intense new film.
For Willis, the role meant transforming himself into a character that was nearly unrecognizable.
"He's an alcoholic. He's overweight. Has a bad leg," he told co-anchor Hannah Storm. "It's fun to put all those things on and see what comes out and see what you look like. It was an image of myself about 10 or 15 years from now."
And while his character is racked by regrets, the character played by Morse is not plagued by issues of right and wrong.
"For me, I'm doing my job, just like we were doing our job all along. He's got to change his mind," Morse said.
What makes the movie so gripping is the triangle of relationships formed by Willis, Morse and the prisoner, played by Mos Def.
"The thing that really makes this thing work is you have these two relationships at the heart of it," said Morse. "You have Mos, and this relationship (between the two cops). And it's so sweet the stuff that happens between these two guys. And there's so much history between us and that's what really is underneath this whole nonstop ride that you take."
The underlying question in the movie is whether people are capable of change. Willis said that off-screen, if there's one thing that has really changed him, it was having children. He has three daughters with his ex-wife, actress Demi Moore.
"After you have kids, everything else becomes stupid," he said. "Everything else that you worry about, 'Can I pay this bill? Can I do this? Can I do that?' … it all changes when you have kids."
Willis also commented on an issue that made some news in recent weeks, involving the discredited author James Frey. After Oprah Winfrey took Frey to task for distorting the truth in his memoir, Willis came to his defense.
"I thought it was a little unfair," he explained. "I respect what Oprah does. I think that her book club is a really good thing and a good project. I just think that he's just a writer, and because of a mix-up or something of his publisher saying these are his memoirs, and not a work of fiction, that he didn't deserve to be publicly crucified."
"16 Blocks" opens on March 3.