Over the years I have found that merely imagining an opponent in front of me is not enough. You really have to visualise this person in front of you, reacting to your moves and striking "him" with intention. When you can do that, it will show in your expression.
Many people nowadays practice Iai by merely going through the motions. Of course it is important to perform the techniques correctly, but more importantly, one should put his/her soul into it. One way to do this is to perfect that cold hard stare, one that will make people quiver.