When it's time for a fight, the cadence of the conversation becomes discrete, and action is handled on a turn-by-turn basis.
Heroes roll for initiative, which determines the turn order in which they take their action. Once everyone has acted, this is called a round, and we roll for initiative again to determine a new turn order. This repeats until there is no longer a need to resolve the combat (the threat to the party is gone, or another game mode is invoked).
Rolling for Initiative
When it’s time to roll for initiative, each player rolls a d6. Add +2 if you have the skill Perception. The GM will roll separately for the enemies as a group (though he may create a separate initiative for special opponents like BBEGs or bosses). Each player and their opponents get to act once on their turn in a the round. Generally, each turn is a few seconds long, and each round is one minute in duration.
After initiative is rolled, the GM goes around the table and ask each player what they're doing on their turn. You can choose to Hold your turn (provided you are not the last person to act), which means you wait to see what others do before you act. If you chose to Hold, you may interrupt the action of those who rolled lower than you, and take your turn before their action is resolved. This is because you're reacting faster in the round than they are, so you can act before they do.
If you roll the same initiative as another character, that means both of you resolve your action on that turn. PCs get priority over NPCs, but all action is resolved simultaneously. This means, if you and your opponent both act on turn 3, and you are beheading your opponent while he's casting a spell, he will complete his spell, its effects will occur, and you will behead him at the same time.
On your turn, you can move within a melee space and take one of the actions below. Your action can’t take longer than a few seconds to carry out, otherwise the GM may penalize you by making you act later in the round. Talking to other characters is not an action, so long as your conversation does not last longer than a few seconds.
You attempt to deliver harm or peril via your weapon or maleficence. If your attack roll consists of more than one attack, such as with the Fighter class, you may roll for each attack and resolve each roll separately. You have to resolve both of your attacks on the same turn you act, though you may reserve one or more attacks and use each to parry later in the round, instead of attacking on your turn.
Use your magic in the form of a sorcery, psionic, or thaumaturgy spell.
Make an opposed attack roll with your weapon, shield, or maleficence: if you succeed, their attack misses. (If you have more than one attack available, you can trade each attack for the option to parry at any time during the round.) You can also sacrifice your shield to parry an attack; this is called a shield sacrifice.
Act in response to someone else's action, outside of your turn (if you have an ability that allows you to do so, such as Counterspell or the kit ability Swashbuckler.)
Move an encounter space (depending on the size of the scene) instead of a single melee space.
Use an item with finesse, such manipulate a magic item that requires activation; equip armor or shields; or read a scroll. Note that most items do not require finesse to use. Drinking a potion, picking up a weapon, or flipping a switch does not require finesse and therefore does not count as an action. Always confer with your GM when in doubt.
Use A stance
Most stances function as reactions or bonus actions, but some stances are actions unto themselves. The stance will always specify when it can be used in its description; if it does not, then it can be used as a bonus action on your turn in the round, or it happens at the same time as the action you take described in the stance.