Adriane Schwager has interviewed thousands of people and hired hundreds in her roughly 20 years in the recruiting space.

She's interviewed candidates at every level in an organization, from interns to senior leaders to join her in the C-suite. Schwager, the CEO and co-founder of GrowthAssistant, a hiring platform, says her most important piece of advice applies to people at any experience level: Show up prepared.

It sounds simple enough, but she says she's noticed that candidates' level of preparedness seems to have declined in the last five years or so.

"I'm actually shocked at how many times people don't do their homework," Schwager tells CNBC Make It. "Here's something very basic that I'm seeing candidates missing right now, which is [understanding]: What does the company that you're interviewing for do?"

Schwager says she'd often give that advice — research what the company does — to college students approaching their first jobs. "But I feel like I'm seeing more of that these days at a senior level, which is interesting," she says.

Her hypothesis is that this tends to happen when she speaks with candidates who have come to her company through a third-party recruitment agency. "Either they're not reading the prep, or maybe they weren't even looking for the role," she says. But, by the time it comes to an interview, she advises doing some research to express your engagement in the opportunity, otherwise it can be a big waste of time.

Gearing up for the interview doesn't have to take too much time. Schwager says it's a plus when a candidate mentions something from her LinkedIn, X profile or other information about the business at the top of an interview.

"That immediately makes me think that the candidate is engaged and starts the conversation off on a very nice note," she says.

If you want to go above and beyond, you might get in contact with any mutual connections you have with your interviewer for feedback on what they're like as a colleague or manager.

One of her most recent leadership hires went the extra mile by listening to podcast interviews that Schwager participated in.

Finally, for some companies, don't discount reaching out directly to the top boss to express your interest in a role. Schwager says some of her most enthusiastic hires have been people who've contacted her directly on social media to express interest in joining her company.

"They know so much [about] why they're applying" and "are always engaged," she says.

Want to land your dream job? Take CNBC's new online course How to Ace Your Job Interview to learn what hiring managers really look for, body language techniques, what to say and not to say, and the best way to talk about pay. 

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