Why should you be acting out your lines, just as a stage- or screen performer would? Your body language (even the smile on your face) can have a drastic impact on your pitch, tone, and impact of your speech. You should address the microphone as you would when addressing a person. I'm sure you would be moving your body, your hands, making facial grimaces and just being real. Do the same at the mic, just keep in mind that you still need to maintain proper mic proximity for recording.
Of course, there are many degrees of acting. Some voice over artists will do very little physical movement while recording, relying almost entirely on voice and facial expression to ensure a great finished product. On the other hand, some will stand in the studio at a standing microphone, so they can move more freely and really put on a performance.
It’s true that many will prefer the final recording of the latter voice over actor. The movement can actually be detected in the voice, providing more realistic vocal expression, and even provides the realistic change in voice that comes with partaking in physical activity. For instance, reading the lines of a ninja, in the midst of battle, is going to sound much more realistic if the voice over actor is a bit winded, breathing a little harder than normal.
Does that mean that you have to stand up and perform for all of the recording you do each day? Not necessarily. Some scripts require less acting than others, and for those sitting may make you more comfortable and therefore more apt to put in a great performance. But, if you have found that your recordings are sounding a bit flat, then it may be time to consider how physically acting out the lines could improve the final product.