I haven't worked with Medecins Sans Frontiers or anyone like that, but I have worked with doctors who have.
Be prepared to attend the worse disasters & war zones possible. One dr I knew went in after the Tutsi/Hutsi (sp?) disaster & the things he described to me would make you sick: dead babies, babies with their heads cut off, hands lying on the ground, torsos with no limbs. Also be prepared to stay in substandard or no accommodation (sleeping under the stars with soldiers standing guard @ night with guns so no wild animals get you), no water or sanitation, etc. Be prepared to really rough it.
Another thing one of the surgeons I worked with told me was that u may have to treat 2 warring parties, one of which may have killed hundreds or more people, and you still have to patch them up 2 send them back to kill more people. You cannot pick & choose who u want 2 treat - you treat everyone 'without prejudice'.
I don't think I could do that if I knew a soldier/general had hacked hundreds of people/babies to death, I just don't agree with treating them and letting them go.