| Treppa |
As long as the opponent meets the requirement of threatening, all the squares he can legally reach are threatened. There is no facing in Pathfinder, so it does not matter if they are in combat with somebody else and you run behind them - they still get an AoO.
But if you move into a square they threaten and make no other moves, you do not provoke. You have to move through a threatened square to provoke attack. A 5' step within their threatened area in place of a move action also does not provoke.
Hope this answers your question.
(Be careful around enemies with reach weapons!)