Karl Henning, US
There are a lot of ways for a character to become captured in a story, but in most epic science fiction or fantasy sagas at least one person ends up in a bubble. It could be an indestructible diamond sphere, a rubbery balloon bouncing across the hills, or just a very resilient soap bubble. Every material has its own set of properties, although each version will most likely be at least clear. Whenever you’re painting something that’s transparent, the main idea is to focus on the highlights and outline.
If you think about your line of sight from a bird’s eye view, you’ll notice that it goes through the least amount of material when aimed directly through the centre of the bubble. Ergo, the sides will be more opaque as the sphere turns away from the viewer (creating an outline of sorts). Even if the character inside is bending their prison into something other than a sphere, just remember that the more perpendicular a surface is to the viewer, the less visible it will be.
I suggest putting the outline on one layer and then creating two separate layers for the front and back of the main surface. This enables you to manipulate their transparency. Place whoever or whatever is trapped on layers between those two and you’ll have complete control over how opaque the bubble is. Then paint in the highlights, keeping in mind that the more shiny the surface is, the more hard-edged the reflections will be.
Since the surface of the bubble is mostly transparent, use highlights and reflections to create a feeling of depth. The thicker and more opaque the skin of the bubble is, the softer edges will be on anything inside.