Windows almost always require a treatment of some kind on Location.
Vinyl frosting, such as Mac-tac can be applied to the camera-side of location glass. Its a good idea to mist the glass first with Pledge so that the frosting doesn't get too glued to the window, especially if the window is in the sun, heat or if its a lengthy install. Its pricy, but it looks really good.
Another option is window sheers; they are a classic look, and if it goes with the style of the set, it can look really classy. Blinds are a great window treatment; however its best to choose the wider slats. Ideally 2" blinds.
Mini-Blinds ans stringy drapes run the risk of moire on camera.
If its a run-down or scrappy look, Scenics can work wonders with baking soda.
Sometimes a window becomes part of the Set. When exposed glass is on a Set it often needs to be gimbaled. Some DOP's can work around the reflections, however it really limits a shot.
Night-time also presents a unique issue on a Location. When its dark outside, the glass acts as a mirror inside.
Shooting Day for Night has the same issues. A Grip can black out a window from ext, however now you've got a big mirror inside.
Textured glass is a beautiful thing. If your Location has it, use it! It
is my favourite obscurity. Especially if it is old, cracked and
Windows are usually an opportunity to do cool things with lighting and window dressings.