If you decide to preach a message “in character,”  then you are choosing to act as someone other than yourself.  People will know that you are acting and to a certain extent they will accept that.  However, there is good acting and bad acting.  Good acting looks real.  Bad acting looks like a performance.  Flamboyant movements and exaggerated speech do not work anymore.  People enjoy movies, tv shows, etc., that seem real.  There is something about “performance” that puts people off.

So when you decide to preach a first-person sermon, try to select a character that can come across naturally.  You may change your voice, your personality, your temperament, but every change puts you at greater risk of “performing.”  Natural communication is powerful, but performance can be counter-productive.  Know yourself, know your listeners and select your character carefully.  Remember, you don’t have to be a character in the story, you could be an observer unmentioned in the text.  For example, Don Sunukjian preached Esther from the perspective of an observer in the king’s court (I suspect he would have struggled to preach as Esther without seeming unnatural!)  Select your character so that you can present a compelling natural account, rather than a contrived and unnaturally flamboyant performance.

First-person preaching is not an excuse to perform, it is a choice to preach a message in the most effective way possible.