First, the virus isn't found in the wild--it's been modified to carry the gene coding for a light-sensitive protein (opsin). When the cells are infected, they start producing these proteins themselves. To do this, the virus is typically applied directly to the brain tissue.
Light causes these opsins to change shape, which then allows specific types of ions to flow through the cell (typically sodium to excite cells, chloride to suppress them). However, for this to work, the channels need to be illuminated directly, with the correct wavelength of light, so an optical fiber is implanted in or just above the brain.
If someone were to do this to you, you would have already had a pair of fairly invasive neurosurgeries, so being mind-controlled is probably the least of your worries! However, this technique only lets you activate all neurons or all neurons of a specific type within the light's path. It would be fairly hard to induce a complex behavior, unless there's a pre-existing circuit for one (as shown here, which is why this paper is so interesting) or they can find an illumination pattern to mimic one.