Morgan Freeman’s oracular voice has become a dependable part of the national soundtrack, so much so that it now substitutes for that of Walter Cronkite, once called the most trusted man in America, on the CBS Evening News. Among Freeman’s most prominent documentaries are the Oscar-winning “March of the Penguins,” “The Long Way Home,” Ken Burns’ “The Civil War” and his ongoing cable science program “Through the Wormhole.”

Still, a sonorous voice alone isn’t necessarily compelling. Freeman’s has a deep, lived-in texture, but it also has a subtle variety of pitches and nuances, from the hortatory (you can hear echoes of Martin Luther King Jr.) to sharp anger, playfulness and flat world-weariness.

“It’s a matter of inflection,” the actor confides. “Reading is just like acting. The content determines which voice you’ll use. I depend on the writer and director.”

“Penguins” is one of his favorites.

“I was so moved by the story, I just went along with it,” he says.

Freeman finds life after god | Freeman’s scientific angle | Freeman’s majestic voice