Find strong directional light - low sun, a narrow alley, a shaft through a gap in buildings. Look for shadows being cast rather than the thing casting them. A long shadow on an empty pavement is more interesting than the person making it.

For silhouettes, expose for the bright background and let your subject go black. Make sure the outline reads clearly.

Watch for: overlapping silhouettes - two people merging into one unreadable shape. Separation matters.

The point: shadow pictures are abstract. Silhouette pictures are graphic. Both pull the viewer in differently.