Yeah, I was thinking of moving it to Tokyo and using Ken Watanabe as Leo, and then keeping my Trapper John and Hawkeye Pierce characters; Americans who are living in a culture they don’t like and they want to get out…I love Sam (Rockwell) to death, but I had Ray Stevenson in my head for a while. Then, obviously, Ken Watanabe. Big guys, because he was going to be silent and I wanted him to have a presence that people could interpret in multiple ways. To one person he might seem dumb. To someone else, intimidating. To someone else, thoughtful. But having him be a big guy was essential.

However…

Actually, Stuart’s first task with us was trying to find out how to film in Japan. Bless his heart, that was the first thing he did on the whole project, and he came back and was like, “Yeah… no.”

From this interview on Deadline.

Not that a futuristic Berlin isn’t the perfect setting for Jones’ fantastic visualization of his unique set up, but the difficulties foreign productions face in attempting to shoot in Tokyo are well recorded and notorious; despite the strong desire from foreign filmmakers. Even James Mangold had to steal the scenes of Hugh Jackman and Okamoto Tao running through the streets of Akihabara for Wolverine like an indie moviemaker.