Every single element within your image is constantly changing when shooting on or through a surface. There are so many factors involved, it's difficult to know with absolute certainty what will emerge within your frame. Such is the beauty of reflections. I've found that one of the most seductive elements of the process is that there's no way to be in total control of the outcome. I'm always delighted by what emerges, especially when it's a pleasant surprise. Many artists call those kinds of discoveries "happy accidents," and I love opening myself and my process up to that kind of serendipitous magic.
So many shifting and changing elements are involved in capturing an image of reflections. First, as previously mentioned, the surface itself will dictate the overall feel of the photograph. Second, your surroundings will directly narrate the story of the layered results. And, lastly, like with most photography styles or genres, light plays a key role when using any of these surfaces in any location or scenario.
The light—with its direction and intensity—is what will illuminate both your subjects and the reflective surfaces so that they mirror back bits of pieces of the scene, whether that be a more clear, even literal representation (like in a mirror) or something more abstract and obscure (like through a window). The more you shoot images using this technique, the better you'll be able to manipulate your surroundings for your optimum end image.
Although capturing these layered photographs can be challenging—the process itself being more of an art than a science—there are the more predicable aspects and outcomes that you can use to your advantage. Getting a textural layered look through a mirror, for example, comes when the surface is dirty—the specks of dust or imperfections actually lay over the subject being reflected back. Or, when shooting into windows, the layers appear either vividly or softly, depending on how each side of the window is lit. Capturing a little bit of what's on both sides of the glass can be one of the most delightful surprises.
The more you experiment shooting into reflective surfaces and observing various results, the more carefully and deliberately you can work to manipulate your image into exactly the shot you want, using a scene or subject matter and its reflection to bring something totally new and fresh to the shot.
One of the most important considerations when shooting reflective surfaces is being aware of everything in and around the reflection, and then creatively framing it. By being conscious of all the layers that the reflection offers, you can place the elements within your viewfinder just like you would compose a shot with any other subject matter.