Slack is more than popular--it's viral. The workplace communication software maker, Inc.'s 2015 company of the year, had 1.7 million users 20 months after its February 2014 launch, making it one of the fastest-growing startups in the world.

Slack's core value is transparency, enabling co-workers to view communications happening across departments and time periods. But at Inc.'s Vision 2020 conference in San Francisco Thursday, Slack CEO Stewart Butterfield said there's another, less obvious component to the company's success: personality.

To illustrate, he related a story that took place earlier this year when Slack went down for a short period. The company responded personally to complaints from users on Twitter, including one who referenced Star Wars: "Work communication has ceased, help us @SlackHQ, you're our only hope."

Slack responded by tweeting the next line from the movie. The customer came back with the third line, and so on, until the two parties had reenacted the entire scene. 

"People want to feel like there's a person on the other side," Butterfield told Inc.'s San Francisco bureau chief Jeff Bercovici, who appeared with him onstage.

Interactions like that between the Slack team and customers--and interactions like that among co-workers using Slack in the workplace--are what makes Slack different as a communications platform, Butterfield said. They provide a departure from the stereotypical dehumanizing, Office Space -, Dilbert-esque workplace.

"Most software just plays right into that, it has no personality," he added.

That being the case, Slack faces a challenge other software makers don't: When your service relies on the personalities of those interacting on it, users need to like each other to like the service. "If you hate your job and who you work with, then you're going to hate Slack," Butterfield said.