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Activities that take longer than a turn can't normally be performed during an encounter. Spells with a casting time of 1 minute or more are a common example of this, as are several skill actions. When you commit to an activity during your turn in an encounter, you commit to spending all of the actions it requires. If the activity gets interrupted partway through, you lose all of the actions you would have spent on that activity.

Reactions in Encounters

Your reactions let you respond immediately to what's happening around you. The GM determines whether you can use reactions before your first turn begins, depending on the situation in which the encounter happens.

Once your first turn begins, you gain your actions and reaction. You can use 1 reaction per round. You can use a reaction on anyone's turn (including your own), but only when its trigger occurs. If you don't use your reaction, you lose it at the start of your next turn, though you typically then gain a reaction at the start of that turn. 

Some reactions are specifically meant to be used in combat and can change how the battle plays out drastically. One example of such a reaction is Attack of Opportunity, which fighters gain at 1st level.

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Attack of Opportunity [reaction]

Trigger A creature within your reach uses a manipulate action or a move action, makes a ranged attack, or leaves a square during a move action it's using.

You lash out at a foe that leaves an opening. Make a melee Strike against the triggering creature. If your attack is a critical hit and the trigger was a manipulate action, you disrupt that action. This Strike doesn't count toward your multiple attack penalty, and your multiple attack penalty doesn't apply to this Strike.

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This reaction lets you make a melee Strike if a creature within reach uses a manipulate or move action, makes a ranged attack, or leaves a square during a move action. The Triggering Moves diagram illustrates examples of movements that might trigger an Attack of Opportunity from a creature without reach and one with reach. 

You'll notice this reaction allows you to use a modified basic action, a Strike. Because your Attack of Opportunity takes place outside of your turn, the attack roll doesn't incur a multiple attack penalty.