It is said that the Highland people go to war over three different things: Land, Pigs, and Women. So as long as you stay away from those three things you should be safe. It is important to consider especially when trekking in the Highlands that every inch of land there is owned by one or other clan who fiercely protect it. It is customary to give money to the people that own the land you are traversing. Therefore it is always best to have someone experienced set up the deal with the locals before setting out. It can also be extremely helpful to have a guide from the area with you to avoid any misunderstandings sometimes a few are needed since not all of them can safely travel through other clans land. The payback skirmishes that happen occasionally is nothing for the tourist to be concerned about. There are even instances when tourists have been able to witness ongoing struggles between families without any harm coming to them, but as always, its better the err on the safe side and stay away from those areas.
talk
As always in Papua New Guinea there is the problem with language. The staff in hotels and some stores will probably be able to speak a rudimentary english but as soon as you get out of the cities people will probably only speak pidgin, or Tok Ples Which is a generic term for the tribal languages. Therefore it's highly recommended for any traveller to bring along a guide that comes from the area.