Any tips for an actor to sound more natural?

Suggest she take it down a notch or two.  

If she asks what that means say match the energy of your scene partners.   Stress that you all need to appear to be acting in the same play.

Work with her one on one to help her recognize when she is being too big.

My wife watches "The Neighborhood" which I can't really stand but I saw 5 minutes of it - maybe just last night. One of the women - "OH....but.....if.....you....want.....to.....do...that......wink,wink,wink." I guess you'd say a pro but certainly 2nd tier.  Supposedly this is what acting schools are supposed to teach, is how to make it effortless. Maybe the point is that if it was easy everyone would be doing it.

When I was in high school in the 1960s, the Drama Club had its members each pick a play to direct, so the more assertive members picked the favorites and I, the shy one, was left with "The Monkey's Paw"---not a favorite among the kids who auditioned.  I decided to make the play an all black cast, then chose a hugely popular gregarious young black guy to play the lead, who helped me cast the rest. Had we finished, this would have won the award---but I was short on organizational skills and inexperienced in the fine art of theater, so we never did get the project completed.  Wish I could do it over again now that I'm 75 and wiser.  

One suggestion I have: Watch the movie "Mr. Holland's Opus" and pay attention to how the character Mr. Holland, the teacher, handled the young overly tense red-headed girl who played the clarinet, but who seemed to falter on the high notes, holding herself back.  Holland had her stay after for a one-on-one session and got her to talk about herself and her family, thinking perhaps that CONFIDENCE was what she lacked.  She spoke of all the ones who were successful in her family, revealing that she was afraid of trying to achieve success and then failing.  Teacher Holland asked this young girl what it is that she really liked about herself and she replied it was her hair---that her dad said it was "like a sunset."  "Play the sunset," he told her.  

This type of approach might work with your actor, Corey.  He or she might be trying too hard while simultaneously fearing failure (riding with one's brake on).  Do some one-on-one listening with your actor and try to find at least one area or line where there is a breakthrough of naturalness.  Catch that moment and have the young actor close his or her eyes and FEEL it---whatever was going on inside that inspired actually becoming the character instead of being outside of it.  Hope this helps.  I'd be interested in knowing if it does help.