Its not complicated, allways protect the D unless you are bluffing and making sure they think you are protecting D 100% for sure (who they dont know) and then protect yourself knowing that. I did that few times as special role player, either D, lawyer, Jailer etc, knew the D and BG, intentionally let in a G in our inner circle msn discussions, work him for few days, convinced him I was BG (the times I was not too), and made sure he got the gist in last minutes I was protecting myself. Generally misinformation purposes it can be useful, but otherwise, as a general rule, allways protect the D, no question. Unless the D is utterly useless player you cant work well with, but generally not the case.