When it's time for a fight or there's imminent danger, the cadence of the conversation becomes discrete, and action is handled on a turn-by-turn basis. You enter encounter mode by rolling for initiative, and the types of actions you can take happen only on your turn in the round, all of which is further described in the rules for Combat & Tactics.
On your turn, you may take one of the actions below (or any one of the actions described in exploration) and move one melee space (this is your movement rate), but you cannot Dungeon Crawl, Give Chase or Flee, or engage in Social Combat that may take more than a single round to complete.
Types of Actions
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 lowering your initiative. Talking to other characters is not an action (and you may talk to anyone at any time during the round), so long as your conversation does not last longer than a few seconds.
Attack
You attempt to deliver harm or peril via your weapon or maleficence.
Cast
Use your magic in the form of a sorcery, psionic, or thaumaturgy spell.
Dodge
Spend your turn focusing on dodging attacks to improve your defense in the round. You confer disadvantage on any attacks made against you in the round.
Run
Move an encounter space (depending on the size of the scene) instead of a single melee space.
Use A stance
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.
Use with Finesse
You can use an item with finesse, such manipulate a magic item that requires activation; equip armor or shields; or read a scroll.
Give Chase or Flee
Elude or pursue a foe. While giving chase or fleeing, you may be required to make multiple checks to complete the chase.
You can also take reactions and bonus actions, which are further described in Combat & Tactics.