Conventional wisdom is that portraits are best captured from about eye level. This is how we see each other in everyday life, and the advice is basically sound from that point of view. However, imaginative images can be obtained from any angle.
For example, the camera can look up from close to the ground, or look down from a raised position. Looking up to a subject gives him or her apparent height and dominance, and consequently an air of authority. Looking down from an elevated viewpoint renders the subject vulnerable or even threatened.
When angles of view are acute, perspective becomes important. Images created from low down tend to show large feet but small arms and head. Those shot from an elevated position show a large head with disproportionately small limbs, adding impact and focusing attention on the face. These are not distortions created by the lens, but an accurate representation of a subject seen in a particular manner. The parts of the body that are closest to the camera simply appear to be the largest.
It is worth training your eye to notice these effects through the viewfinder. They can be used to direct attention to particular features. Try shooting up from beneath the chin of a reclining figure and, using a wide-angle lens, from between the feet of a seated subject. The back of the head is definitely worth investigating, particularly if your subject has an unusual hairstyle or perhaps a bald patch. Hands clasped behind the head add appeal although may become the centre of interest, and the diamond created by the arms makes an interesting frame.
All sorts of circumstances can lead to the choice of unusual viewpoints. Unwanted backgrounds can be reduced or eliminated by gaining a little height and looking down on the subject. If we shoot upwards the foreground reduces and the sky becomes the backdrop.
Alternatively, an image can be transformed by moving around the subject so that light strikes from different angles. Front lighting reveals detail and colour but can be uninteresting. Viewed against the light, and exposing for background highlights, the same subject becomes a silhouette. Colour is then sacrificed in favour of shadows and strength of outline. Colours and shapes in backgrounds can also be manipulated by changing viewpoint. Contrasting colours might, for instance, be replaced with colour harmony by taking a couple of steps to one side.
Unusual camera orientations can also be exploited. For example, tilt your camera to capture a diagonal cut through the scene. This will give the image a quite different and less formal feeling. Once again, the key is experimentation.