I recently shot a seated 1 plus 1 interview in an art museum. I expected the sound to be an issue do to try and reduce the echo I used two sennheiser 416 mics on mic stands positioned above the heads of the participants as close to their mouths as possible. However there is still a considerable echo on the recording. Is there another way to shoot an interview in such a location to dramatically reduce the echo or is it simply a case of don't shoot an interview in a big open room with hard walls and floors??
I would probably go with lav mics. Not a perfect solution, but depending on the pattern, it will make a moderate improvement in most situations.
Get out in the middle of the room, and stay away from the walls. Also look for acoustic reflectors like corners and curved parabolic walls pointed at your set and reposition yourself if you can to avoid the reflection path. Be sure to look up, and check the ceiling too.
Building a sound tent that is just off camera is another idea. Depending on how much time you have, it could be elaborate, or simple. Furniture moving pads are cheap, you can put them on the floor, on c-stands off to the side, behind, etc. Those upholstered office cube partitions that can stand on their own can also be used to do this, but the moving blankets are a lot easier.
In the end there is only so much you can do with these type of big rooms. You could avoid them by shooting background plates of the location, and doing a green screen shot in a location that has better acoustics.