Actions in Dungeons & Dragons 3.x and Pathfinder come in a wide range of types. Not-an-action, Free, Immediate, Swift, Move Equivalent, Standard, Full-Round… I think that covers all the ones that come up. Some… things done by characters don’t fit the round-based action model, such as spells that take more than a round to cast, but in combat at least, this is the list.
I’m going to take a tip from FantasyCraft and reduce the set a little.
Not-an-action can go. These are things that don’t warrant action cost, such as shouting something brief and not waiting for a response. Never mind, call it a free action.
Free action is on parole. There aren’t that many things that honestly should be ‘free’ when time is being measured.
Immediate and Swift. I’m certainly keeping Swift, but I think I’ll get rid of the distinction of Immediate actions. I think I may abstract the mechanism so that some things may be done as interrupts but will cost actions from the next turn. HERO System allows you to abort to certain actions, which may include full standard actions. For instance, evasion might be replaced with the ability to react to an area effect by getting out of the way, but it costs either a swift action (you just had to shift position a little, a five-foot step) or a standard action (you had to move further, up to your normal movement rate).
Move Equivalent is gone. Low-level martial characters get a single attack per round whether they move or not, so what benefit is there to a full attack action? Let them have two standard actions. It may be necessary to do something with casting times; spells scale much better with level than martial attacks do, so doubling those for free will be bad. Spell casting time may go up to “two standard actions” for most spells. It’s something of a nerf, of course, but I think I like how it will work.
Standard action stays. Attack, cast a spell, whatever. Some talents will allow a character to do more than one related thing per action (such as attacking once with each weapon as a single action).
Full-Round actions go. Some activities will require multiple standard actions, but you can split them across rounds if you want. You may be more subject to interruption, of course.
A character gets two standard actions and one swift action per round. Most things that can be done cost a standard action, some that are very fast cost swift actions, some that are slower may take two or more standard actions. Actions that cost more than a standard action may be spread over two or more rounds, using consecutive actions until complete.
Normally a character may only spend actions during his turn. Some situations outside a character’s turn may allow him to spend actions from his next turn to respond. For instance, an action that provokes an attack of opportunity allows a character to spend a swift action from his next turn to make a simple attack against the provoker, during the provoker’s turn. Depending on talents, the responder may have other options that might use standard actions.
It may be possible to gain additional actions per round, either standard or swift. Some talents will allow you to do more with a single action, such as Expert Two-Weapon Fighting allowing you to spend a single standard action to attack once with each weapon.