Source: Ginny & Jeff Schuett for 
A good defender must know the difference between active and passive defense. An active defense is one where you attack, sometimes taking a risky approach in order to establish fairly quick tricks for your side. This is particularly indicated when the dummy has a long suit (or suspected long, strong suit from the bidding). The defense needs to establish their tricks before declarer can discard losers on that long suit.
A passive defense is where you try not to break new suits in an effort to not set up secondary honors as winners for declarer. Some examples of considerations when thinking whether to return partner’s suit:
1. If partner leads the queen as top of a sequence, dummy has small cards and you hold the A, you know declarer has the king and if you return the suit, declarer will win with the king, and now controls the play. If dummy started with a doubleton in a suit contract, declarer can ruff other cards in the suit in dummy; therefore returning partner’s suit is passive and mostly helps declarer. If a side suit looks appealing, shift to it NOW.
Contract = 4
Partner leads 
Q implying a sequence of QJ10x or similar. It is critical to win A and shift to 
J to attack dummy’s weakness.
2. If partner leads a suit fairly short in dummy and it appears that dummy’s primary value is having that shortness and the possibility of providing ruffs for declarer, win the lead and consider shifting to a trump to cut down dummy’s ruffing value.
Contract = 4
Partner leads 4 implying heart length. From your minor suit honors, it looks like the minors will not provide discards but the heart shortness in dummy might provide ruffs for declarer. Therefore win the A and lead a spade to minimize declarer’s ability to ruff hearts in dummy.
3. If partner’s lead has set up a series of winners for declarer, unless you think they have a singleton, consider shifting to dummy’s weak suit.
Contract = 4
Partner leads T implying a sequence of 109x or similar. It is critical to win the A and shift to a diamond.
Although you would usually lead low, it is best to lead the J in case partner has something like AQ10 and can cash three diamonds immediately.
In summary, look at dummy and try to see if attacking or being passive is a better approach. If attacking, lead top of a sequence or honor if desperate. If it appears declarer is trying to set up ruffs in dummy or dummy’s best asset is shortness, consider leading trump which is usually the best defense to minimize the ruffs declarer can get later in the hand.