Understanding the different types of actions available to characters in the Adventure System is critical for the referee and players. Properly using actions in different situations brings forth emergent behavior built into the rules that provides a meaningful in-game distinction to different elements of the game. There are three different types of actions: Free Actions, Standard Actions, and Complex Actions. This reflects the time required within the context of a turn to perform the action. Typically, a character gets two Standard Actions or One Complex Action per turn. They also get one Free Action per Standard Action per turn; usually two, but if the character were reduced to one action due to being Stunned, their available Free Actions would also be reduced to one. Free Actions often don't require a skill check, but can be used at any point during the turn. This is described in more detail on Page 39 of the Core Rules.
Different combat options are also categorized into two categories: Basic Actions and Advanced Actions. Attack, Defend, Disengage, and Move are Basic Actions. More involved tactical options are Advanced Actions including things like Charge or the use of any divine, magical, or psychic power. There is no hard line between the type of action (Free Action, Standard Action & Complex Action) and the category of action (Basic Action or Advanced Action). The former represents an element of time while the latter speaks to the character's status in a fairly simple manner. A character that is Shaken cannot perform Advanced Actions. They can only take Basic Actions until the condition is removed. This very important to any character with powers. If they become Shaken, the can no longer access their powers until the condition is removed. This restriction is used in the Adventure System in place of a limit on the number of times the character can use their powers.
The type of action required to ready a weapon is a great example of how actions are used to create a meaningful distinction in the Adventure System. As long as the referee is paying attention to whether the characters have weapons ready or not, situations will develop organically that will highlight the benefits of a light weapon as compared to much harder hitting heavy weapon. If this type of thing is ignored, then the downside of using a heavy weapon never comes into play and it can become unbalanced. If they want, the referee can also craft specific encounters with this in mind.
As an example, let's look at an encounter with a band of goblins. The characters consist of an elf armed with a short sword and dagger (both light weapons), a mage armed with longsword (a medium weapon), and a dwarf armed with a battle axe (a heavy weapon). The characters are travelling through a darkened forest where the goblins wait in ambush and none of them have their weapons ready. Fortunately, the sharp-eyed elf spots the goblins and the characters are not surprised, but the goblins win initiative and attack.
The goblins use Charge allowing them to move and attack as a single Standard Action. This is an Advanced Action and confers Disadvantage to their attacks due to having moved as part of the charge. It leaves them with a Standard Action remaining to defend if the characters attack.
Because readying a light weapon is a Free Action, the elf can draw her weapons on the goblins turn. Even though Free Actions are reactionary, she can draw her blades as the goblin closes because it moved and is ready when it engages and attacks. It is important to note, an unarmed defender has Disadvantage to defend against an armed opponent. Not only is the elf no longer unarmed, but once she has her dagger in hand she could use Preemptive Strike to throw it at the goblin as it closes. The attack would have Disadvantage against a moving target, however, so the elf maintains both of her Standard Actions letting the goblin attack. The goblin hits and she uses one of her Standard Actions to Defend thwarting the attack. With one Standard Action remaining the elf can now attack the goblin on her initiative.
Unfortunately for the mage, readying a medium weapon is a Standard Action. With the exception of the Defend Action, which can be taken when attacked, a Standard Action can only be taken on the character's initiative so the mage cannot draw his sword and is an unarmed defender. Now for some magic. Knowing he can't draw his weapon in time, the mage casts Shield which can be manifest as a Defend Action. The use of the power is an Advanced Action and requires one Standard Action. Shield adds half the mage's Spirit to his Defense resulting in an adjusted Defense of a 7. With a Fight d4 and Disadvantage, the goblin has little chance of hitting against a 7 and fails. The mage now has one Standard Action to use on his initiative. He could use it to draw his sword, but that would leave him with no actions remaining to attack. He could attack unarmed, but that would grant the armed goblin Advantage on his defense. The mage also has the Witch Bolt power, but it requires two Standard Actions to manifest. It is important note, this is very different from a Complex Action. A character only gets one Complex Action per turn in place of both their Standard Action and it must occur within the confines of a single turn. Witch Bolt on the other hand, requires two consecutive Standard Actions by the caster. The actions do not have to occur in the same turn as long as the caster does not take another action in between. Relying on the protection of Shield, the mage uses his remaining Standard Action to begin casting Witch Bolt. If the goblin attacks on the next turn and hits, the mage has two choices. If he uses a Defend Action, his attempt to cast Witch Bolt is interrupted as the two required Standard Actions can no longer be consecutive. He could also just take the hit, but if the goblin's damage check equals or exceeds the mage's Toughness he will, at a minimum, be Shaken and may suffer wounds. If he is Shaken, he also can't finish manifesting Witch Bolt because using a power is an Advanced Action.
The dwarf has far fewer options. It requires a Complex Action to ready his battle axe, a heavy weapon. Goblins are cunning and wicked creatures. Let's say that this one, seeing the dwarf's axe strapped across his back, decides not to Charge. Instead, he uses one Standard Action to move engaging the dwarf and the remaining Standard Action to attack. Because these are separate actions, the goblin suffers no Disadvantage to attack. If he hits and the dwarf uses a Standard Action to defend, the dwarf can't draw the axe because a Complex Action will no longer be available. Perhaps worse yet, the dwarf would have Disadvantage to defend as an unarmed defender. As long as the goblin keeps hitting, the dwarf will be faced with the choice of taking a hit or waiting for the goblin to miss before being able to make ready his axe.
While both the dwarf and the mage potentially faced a decision to take a hit rather than defend, their circumstances are very different. The mage is likely to be lightly armored and, therefore, not often inclined to take a hit. Even if the mage was only Shaken, that is enough to cut off his ability to use magic. If the dwarf is heavily armored, he might be more than willing to take a hit from a lowly goblin. A blow from an orc or an ogre would be another matter entirely. Perhaps, the dwarf will start carrying a light axe as both a potential thrown weapon and backup in circumstances like the one presented. That can lead to an interesting fight where the dwarf can't use his preferred weapon.
The action cost of a power is extremely important in coloring how and when the power is used along with creating variety in different types of casters. Once combat begins, powers that require one Standard Action operate very differently than those that require two or more. The Quickening talent is defining for a caster. It reduces the number of actions required to manifest a power by one including reducing a power that requires one Standard Action to a Free Action. It is, however, an expensive talent. It is unlikely that a character with only has a few powers that round out their concept would take it. For example, a ranger or a bard with a few elemental powers isn't likely to spend 25 Destiny on Quickening. The cost would buy two different advances and half of another. Similarly, a typical wizard who often has the luxury of being defended by his companions, allowing them to cast unthreatened, is not likely to bear the cost as they could instead have another a Minor Magical Power and a Magical Power or even a Major Magical Power. But to a combat caster, who relies on magical strikes for their offence, it will be indispensable.