Conduct a warm-up activity with the class that will free the students up vocally (especially articulators and resonators) and physically (especially tension areas such as the neck and forehead and jaw).
Give the students some time to run through their monologues or scenes.
Have the students perform their previews. While they are performing you will take notes on the evaluation sheet. You can have the audience students respond verbally after each preview or assign some kind of observation writing assignment to each preview.