Actions, Bonus Actions, and Free Actions, and Flanking.
Actions
Generally Rules As Written, unless a specific item mentions otherwise.
For example all potions to consume or administer to somebody else is still 1 Action, Donning/ Doffing Shields..
Bonus Actions
Again as written.
Free Action
Standard 1 Free Object Interaction
Bonus FREE Action (no cost to use)
Talking, on the player’s turn, keeping iit short, (in combat, the player will need to wait for their target creature to respond, once it’s their turn).
Dropping an object to the floor.
Not an Action or Using of Movement,
Sitting, Kneeling, Laying Down, not spending an action, and not using movement, a player can go into this position (in combat) and will be classed as Prone, without any cost.
Rules As Written example:
It is still a Free Object Interaction to pick up an item off the ground, and an Action to equip an item in the other hand like a weapon or a shield, in that 1 turn..
But in that turn, you could also speak, drop an item, and sit down.
Flanking,
An optional rule that is activated,
Reactions
As written too.