Maureen Sullivan is no stranger to success. And as chief operating officer of online rental clothing company Rent the Runway — with stints as an executive at major corporations like Google and AOL — she's learned that there is a particular skill that's crucial.

"I think nothing is more powerful in business and in life and overall, than confidence," Sullivan tells CNBC Make It.

But it's not just about self-assurance. "I think confidence comes from preparation and deep expertise and really knowing what you're talking about," she says.

"I think that for anyone who is trying to accelerate their career, and really have a stronger voice within their company, that confidence is key."

Rent the Runway launched in 2009 and soon became a darling in the e-commerce space. The business allows customers to rent designer apparel and accessories for a fraction of what it would cost to buy them. For a monthly fee (ranging from $89 to $159, depending on how many pieces you want to rent), members can pick what they like each month and then swap out those items for new ones.

The company did face criticism around its workplace culture after several executive departures in 2015 (Sullivan started that October). At the time co-founder and CEO Jennifer Hyman told Fortune, "Like any startup in hyper-growth mode, growth often brings change, and with it, evolution in the executive team." Rent the Runway also has more competition these days, with businesses renting everything from clothes to bags to bridesmaid dresses.

Still, to date, Rent the Runway has raised $210 million, with a valuation of $770 million, according to CNBC. The service has over 9 million members and over $100 million in revenue, CNBC previously reported.

As for Sullivan, with her position in the C-suite, in addition to confidence, there is another skill that has served her well.

"You have to be detail-oriented," Sullivan says."I think you have to be able to have a big vision, but really obsess on every little, minute detail. And I think what I love about my job at Rent the Runway is our vision is massive, but how are we going to get from point A to point B and all the way to Z?

"And that obsession with the work and how we're going to do it in a way that's efficient and innovative and constantly challenging ourselves to get better and to scale, makes my job extremely awesome," she says.

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