Examples 1-3 in the image gallery illustrate, in an non too subtle way, the effects of over, under and correct exposure. Over-exposure causes loss of detail in the bright highlights. In this example it is especially noticeable in the actor's upturned face, as he look straight into the light. Detail and edge definition is also lost from the happy/sad faces on the proscenium arch.
In the case of the under-exposed image, the effect is of a murky, dull overall appearance, with few bright areas and loss of detail in the shadows, particularly noticeable in the indistinct boundary between the actor's hair and the gloom of the darkened stage behind him.
The third version walks a fine line between correct and over-exposure. I have allowed the highlights on the upturned face almost to burn out, in order to emphasise the intensity from staring up directly into the light.
The techy details of this shot are: Nikon D300 with 28-70mm f2.8 zoom, at 70mm. The ISO was 2000 giving an optimal exposure of 1/25 second at f3.2, manually metered. I shot this production using a tripod rather than my usual monopod, as the light levels were very, very low through out.