Filmmakers asked to shoot the finale during the real event in 2017, and organizers decided to let them do it. The result begins streaming November 22.
Making a feature film during this marathon requires a lot of planning and a lot of cooperation between race organizers and moviemakers.
"You can't do it unless both sides are committed to making sure it doesn't get in the way. That it's just part of the action," said Chris Weiller, senior vice president of New York Road Runners.
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In the movie, when we first meet Brittany, she is overweight and out of shape, but the film shows her transforming her life through determination and hard work. The real Brittany said that she "started as a runner very late in life."
"So me discovering what running is, being a beginner as an adult, was a real challenge," O'Neill said.
Jillian Bell plays a character based on O'Neill in the movie. The shoot was so carefully planned that fiction merged with fact at the finish line.
"It's amazing. When they came through the finish, and she was struggling over the finish. It was a big part of the movie, the runners thought Jillian was just another runner, and they were helping her," Weiller said.
O'Neill said actual marathoners tried to help her across the finish line.
"And the director was like, 'No, get out of the shot!'" she said.
It was all part of getting greater realism on the screen. O'Neill thought it was essential to shoot at the real event.
"There's nothing like it: the energy, the inspiration. You can't recreate that kind of energy, and so having them film during the actual marathon in New York City was huge," she said.
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Much of what's shown is fictional, but "Brittany Runs a Marathon" captures the spirit of the race and the herculean effort required to finish. The woman who inspired the film is happy with the result and hopes "that it inspires more people to run it that wouldn't think that it's a goal they can set for themselves."
O'Neill said it's "definitely bizarre" to see yourself depicted on the big screen. The experience was "bittersweet" for her because she can no longer run in the TCS New York City Marathon due to injuries, but she will be there cheering on Sunday!
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