The Coca-Cola Company on Sunday night is placing a lot of weight on Hayley Magnus’ shoulders.

The Australian TV and film actress was supposed to shoot a six-second video that Coke executives planned to use for social media support in a big campaign for Diet Coke. She was supposed to do something funny with her shoulders in front of a giant yellow wall on a set in east Los Angeles. Instead, Magnus ad-libbed her way into a Super Bowl commercial.

“She was so funny from head to toe doing this dance,” recalled Paul Feig, the creator and director behind “Bridesmaids” and “Freaks and Geeks,” among other works, who directed a new group of Diet Coke ads. And she started to narrate her action, which wasn’t necessarily in the script, he said. “She was getting sillier and more charming. She’s ad libbing. I go, ‘I can’t stop!’ We were just cracking up, and I said, ‘I think this just might be the Super Bowl spot.’”

The 30-second commercial – the first time Diet Coke has appeared in the glitzy TV event in 21 years – was slated to air at 7:30 p.m. during NBC’s broadcast of the game.

Coca-Cola, like so many big advertisers, typically leaves little to chance. Some of its executives visited New York City last week to hold meetings with small groups of reporters to explain in great detail a separate Super Bowl effort behind the company’s flagship drink, Coca-Cola.  But as a walk-through of the process behind the Diet Coke commercial shows, sometimes a company must let things proceed unfettered.

“We chose Paul as our director because he has this wonderful ability to pull improvised and comedic performances out of actors. You can see that in all his work, and I can tell you only one of the shoots ended up as scripted,” said Danielle Henry, group director of integrated content for Coca-Cola North America. “There was a tremendous level of improvisation, and I think that those hit the groove.”

The company has big plans for Diet Coke. A rising generation of consumers is eager to drink more healthily (and in some cases, avoid artificial sweeteners) and test new flavors. With that in mind, the company has expanded Diet Coke’s palette. Yes, the original continues to be sold, but consumers can also try Ginger Lime, Feisty Cherry, Zesty Blood Orange and Twisted Mango (in the Super Bowl ad, Mangus tells viewers what mango makes her want to do). The drink is served up in taller, sleeker cans.

Coca-Cola spent two years considering the revamp, talking to more than 10,000 people about flavor ideas and packaging.

The changes come as diet sodas have seen sales fall.  In 2016, sales of Diet Coke dipped 1.9%, according to Beverage Digest, while sales of Diet Pepsi were off 7.2%.

Coca-Cola wants to reverse the trend. Once portrayed in commercials as “hyper glossy” and “feminine,” says Henry, the company executive, Diet Coke in its new campaign is aimed at younger consumers of all kinds. The Super Bowl gives the soda giant opportunity to put the new concept in front of millions of people from all walks of life. Coke wants to keep Diet Coke loyalists, she said, but “we are looking to expand our reach.”

If Magnus is aware of all the work and research behind the Diet Coke effort, she certainly doesn’t show it. Feig says no one on set felt any pressure. “There are some Super Bowl commercials you just know are going to be these giant projects. It’s ‘This is the Super Bowl. We’re going to come in blasting.’” he said. “All that weird pressure was off.”

Will viewers see more of Magnus after the Super Bowl? Coca-Cola’s Henry believes so. “We think she’s going to go pretty far,” she said. That’s certainly not a shoulder shrug.