Bluffing is generally thought of in relation to playing poker, not chess, but it does apply to the game of chess albeit ineffectively applied at times. Many opponents will attempt to place pieces in an open space on your side of the board with no real intent of sacrificing the piece. This is particularly true towards the beginning of the game when your opponent is trying to feel you out. An opponent may run a bishop or knight out to your side of the board as an attempt to establish an offensive front.
If you ignore this piece and allow it to sit out there while executing your own plan, then you are giving your opponent an advantage. Find out if your opponent is simply bluffing and trying to feel you out. Many times if you challenge this piece, your opponent will retreat quickly instead of standing to fight. Similar to a game of poker, your opponent wants to see what type of player you are. They are trying to see if you are an aggressor or a protector of your pieces. Call this bluff from your opponent to find out how badly they want to keep this piece out in the middle of the board.
If you simply allow this piece to sit in waiting as an aggressor, then you are giving your opponent the advantage. In a worst-case scenario you will simply trade a bishop for a bishop, or a knight for a knight. This is still a good protective move by you because leaving that piece sitting out there unchallenged will prove to be a thorn in your side throughout the game. Challenge every piece that comes onto your side of the board and determine how strongly your opponent feels about keeping that piece there, or if they will simply cower and retreat.