Famitsu conducted an interview with Kojima Productions developers Junji Tago (technical director) and CG art director Hideki Sasaki. Gaming website Polygon posted a translation.
The reason Kojima Productions decided to make their own engine, was that it would allow them complete freedom in game design. They didn’t want to be limited by features in existing engines, since they are trying to make something that goes one step beyond normal games.
One thing that makes the Fox Engine stand out from other engines is the fact that the changes you make in the game immediately appear on screen. This saves times and allows for more trial and error, and, eventually, more quality.
While the engine team and the game development team are seperate, they do work closely together. Hideo Kojima may request certain things, which then get implemented by the engine team. At GDC, they talked about aiming for photorealism in Metal Gear Solid V, but at the same time, artistic design is very important as well.
With Metal Gear Solid V, they are working with a combination of body and facial motion capture, instead of doing perfomance capture (which is doing it all at the same time). The difficluty of performance capture is that one actor has to do everyting (face, body, voice) right at once. This would lead to more retakes. Another option would’ve been to do go to Hollywood for the recording, but that would have meant Kojima going back and forth all the time. So they decided to record voices and body seperately.
Fox Engine is meant for wide usage, more than just MGS. Right now Konami is developing a new Pro Evolution Soccer using the engine.