Most of the interiors of the buildings in Watch_Dogs can’t be visited, but when you’ll look at them from the outside you’ll actually see them through the windows. You can see the effect in the animations below (you can also find a higher quality version here).
What many don’t know is that those interiors actually aren’t there, or better, they aren’t rendered in 3D, but use a visual technique based on a real-time reflection shader called Interior Mapping. A few other games used it in the past, but never with this level of fidelity and this extensively.
If you want to read in detail how the effect works, you can find a technical explanation from SIGGRAPH here.
What’s most important is that it’s a large step forward in immersion and visual quality compared to the flat textures windows we see in many games, giving the city a much more realistic look with an almost negligible impact on performance.