The defenders default to passive play. When they don't know what to do, which is most of the time, the defenders react to declarer's iniitiative. If the cards lie well for declarer, the contract will succeed.
On some occasions, though, one defender, or perhaps both, will realize that an active defense is necessary. The defenders' pathway to knowledge is to count declarer's tricks
The defenders build up their knowledge of decarer's hand by listening to the bidding, counting cards as the play progresses, and drawing inferences from declarer's choice of plays. In the absence of compelling information, the defenders should avoid any action that breaks a new suit or has the potential to give declarer a trick.