I was fortunate to meet R. Lee Ermy a few weeks ago, at a Glock Inc. tradeshow booth. Had him sign a VHS "Full Metal Jacket" copy for me.
I asked him about the dialog in FMJ, he said he was surprised when he saw the final edit, and it came out better than he had thought possible. He also said he wrote the dialog for his part, something I was not aware of.
His DI is easily one of the most compelling characters ever portrayed on film, and Full Metal Jacket is one of the top five best selling movies - VHS and DVD -of all time.
All he had to do was act naturally.
Heheh I got to see a special or something on Full Metal Jacket and they talked on how he came up with those lines especially when he is yelling at the recruits.
Basically what he did was get in some poor shmucks face ( I take it someone who worked for the director or writer) and start yelling at him like a Drill SGT. and they had a tape running... then they took the dialog almost verbatim from that tape and put it in the script...
Emery is one cool dude!
FMJ, IMO, is one of the most compelling movies ever made. The dialogue is tighter than a clam's *ss, the acting is first rate, the music is awesome and the direction is, well, Kubrickian.