They’re in your space, in your face, in your nightmares — but screaming aside, what’s really going on in the heads of Marine Corps drill instructors? Drill instructor duty is no joke. Whatever recruits do, DIs do. And the Marines on the three-year-long special duty assignment set the example for what it takes to become part of their Corps. That means yelling the loudest, moving the fastest and having the most squared away uniforms. Marine Corps Times was afforded access to drill instructors aboard Marine Corps Recruit Depot Parris Island, S.C., recently to find out some of the secrets behind the…