I wish I had a nickel for every time I saw one of the two words above in an audition spec. If you go to You Tube and click on any of the TV commercials of yesterday, say back in the 50's and 60's, you're more than likely going to hear an announcer type delivery that sounds over projected with volume, and a lot of "sell and hype" in the voice. Not to say that those don't still exist in today's advertising world ( i.e.-Hard sell auto commercials or infomercials). Now more than ever however, casting directors are looking for voice talents who can sound natural (non-hyped/ conversational), and believable, while standing or sitting in front of a microphone. That takes good old fashioned experience.
I have found newbie voice talents tend to often make the mistake of speaking too loudly into the microphone. This results in an affected and over projected voice quality that is far from sounding natural. The key is to let the microphone do the work of amplifying your voice, and "get small" with your voice-less volume. Imagine that your best friend is standing right next to you and you're speaking to him or her. You certainly wouldn't get overly loud in their ear when speaking, so why do it when recording a voice over? Another analogy would be people who feel the need to yell into their cell phones, as if the listening party cannot hear them.
Quick tip: Recording with headphones off (or maybe just one ear outside the phones), can help you to achieve a more natural sound. What happens is many folks get too caught up listening to themselves through the headphones, which can result in an unnatural sounding voice over. I realize that in studio settings where producers/directors are giving you direction from outside the booth, you'll need to have them on to hear. But if you're voicing from a home studio and self directing, try taking the headphones off sometimes. I think you'll find that helps to make your reads more natural. Still, there are many experienced voice talents who can sound natural with headphones on while recording. My advice so far is mainly intended for those just beginning their voice over endeavors and struggling with this issue.
Another part of making the voice over sound natural and believable is not overly enunciating the words. If you're voicing a "real person" type script , you'll want to aim for more conversational diction. Pronouncing ev-e-ry lit-tl-e syl-la-bl-e with over emphasis will make you sound very stilted, unnatural and amateurish. On the other hand, you're not aiming for sloppy either. You have to kind of split the difference. Grab those audition scripts and practice! Sounding natural while in front of a microphone is an acquired skill.