There were other times when he needed more energy out of Nagai, especially when the youngster had to repeat a line dozens of times.
Nagai looked up at Docter: "Yeah," he said in barely a whisper.
Because Nagai is so quiet, it was suggested that Docter do the interview with him at a room poolside at the Hollywood Roosevelt recently. But the filmmaker still ended up doing most of the talking.
Nagai was chosen from about 500 kids who auditioned to play Russell, a typically rambunctious 8-year-old who befriends a crotchety, elderly widower named Carl ( Ed Asner). After Carl's wife, Ellie, dies before they got a chance to travel to the legendary Paradise Falls in South America, Carl, who worked with balloons all of his life, attaches thousands of balloons to his house so he can travel to the location. Russell ends up stowing away in the floating house.
"I don't know if we told you this, Jordan," Docter told Nagai, "but in some of the later sessions your voice had gotten a little bit different, so we had to pitch it up to make it a little bit higher so it would match your earlier voice because you are getting big."
Originally, Nagai's older brother Hunter was to audition for Russell. But Nagai, who is 18 months younger, tagged along. "I have done auditions before for commercials," Nagai said. "But not for a movie."
While at the session, the casting director asked Nagai to try out.
"You got to go up and say some things about judo," Docter said to Nagai.
"They recorded Jordan here, and they brought the tapes up to Pixar and we listened to like 35 tapes. As soon as Jordan's voice came on we started smiling because he is appealing and innocent and cute and different from what I was initially thinking."
After doing more auditions and recordings, Nagai got the job.
Initially, said Docter, Russell was to be more hyper. In fact, the initial concept was based on the effusive personality of animator Peter Sohn, who was the voice of Emile in "Ratatouille" and is the director of the short "Partly Cloudy," which will be shown before "Up."
"But Jordan, you were a little more calm and relaxed about things," said Docter, who also directed Pixar's "Monsters, Inc." "We started to change the character, and that made him more unique and special."
Since completing "Up," he also supplied a voice on an episode of "The Simpsons," which will air in December.
Though he never recorded his dialogue with any of the other "Up" actors, that wasn't the case with "The Simpsons."
"I was with the person who plays Bart [Nancy Cartwright]," Nagai said. "She said I was the first kid to record on 'The Simpsons.' "
Was Nagai surprised to learn that bad boy Bart is actually played by a woman?
"Kinda," he said with a small smile.