If you are sitting next to someone snacking on a tub of buttered popcorn, you might miss important details in Steven Spielberg’s epic film, “Lincoln.”

So lean in during the scene in which Daniel Day-Lewis, starring as President Abraham Lincoln, sits in the dim light of his White House study deep in thought – a man bearing the weight of a torn Union while devising a political strategy to persuade the House of Representatives to pass a 13th Amendment to the Constitution that would abolish slavery.

A gold watch swings across his desk, cutting the shadows.

Listen closely for the loud beat of the swinging pendulum.

That pure, unadulterated tick is the sound of an original watch that Lincoln carried.

“I heard the actual pocket watch existed,” Spielberg said in an interview, “and I wanted to know whether they’d let us wind it and record it. I didn’t know if they would, and they did. I thought that was very important. So, every time you hear that little ticking in the story, that’s Abraham Lincoln’s actual pocket watch.”

Spielberg dispatched a team to find other sounds that surrounded Lincoln in his final days.

“We don’t often recognize authenticity as a dimension of sound,” said Ben Burtt, an Academy-Award-winning sound designer who conceptualized the audio effects in “Lincoln.” “You want the audience to believe the sounds are authentic.”

After Burtt read the script, he began to consider which original sounds could help bring the movie to life.

Burtt discovered that one of Lincoln’s pocket watches was in the collection of the Smithsonian’s National Museum of American History. “I asked the question, ‘Has anybody wound it up to see whether it would tick?’ They said no. … I tried to encourage them to do this. There was a certain level of risk involved with tampering with the watch. Eventually, they bowed out.”

Burtt kept searching and found another watch that belonged to Lincoln at the Kentucky Historical Society in Frankfort, Ky.

Greg Smith, who teaches film at American University, traveled to Kentucky to record the watch for the movie. Smith built a special felt-lined box to block outside noise. “We did a dozen recordings of the same watch” to capture a perfect take, Smith said.

Smith also recorded the ringing of the steeple bell at St. John’s Episcopal Church, the pale yellow church across Lafayette Square from the White House, as well as the squeaks from the “Lincoln Pew.”

As president, Lincoln would walk across the field from the White House to the telegraph office, where he would read war reports. Then, he often would walk to the evening service at St. John’s.

“He was always alone,” said Hayden Bryan, executive director for operations at St. John’s. “The president would arrive quietly and leave before the services ended to walk back to the White House. He would sit by himself. He had no aides. No security. Before the service ended, he would get up. The worshipers didn’t even know he was here.”

Bryan sat in the Lincoln pew while Smith recorded squeaks from different angles. Smith also recorded Bryan stepping across church floorboards. “These are the actual floor boards Lincoln walked on to get to this pew,” Smith said.

The recorded sounds from the pew are used in the movie whenever Day-Lewis sits and rises from a chair.

Burtt and Smith also recorded inside the White House. “The thing we were there for was the clock on the mantel in Lincoln’s executive office,” Burtt said. “The clock is still there. It is a French clock purchased during Andrew Jackson’s administration.”

The sound of the clock “is used in the movie in many scenes in Lincoln’s office,” he said.

Other audio effects collected from the White House included the sound of the latches as doors opened and closed, and the sound of someone knocking on those doors.

At the Studebaker National Museum in South Bend, they found the elegant barouche carriage that Lincoln took to Ford’s Theater on the night of his assassination.

“We recorded the door opening and closing,” Burtt said. “We recorded the squeaky suspension.”

The authenticity of the sounds adds to the movie’s drama. As the tension rises, Lincoln’s watch can be heard clearly ticking away history.

“I could have recorded a watch that belonged to my great-grandfather, and the audience would have accepted it,” Burtt said. “But there is something sacred about working on a film about Lincoln.”