How does the amulet wearer control a Shield Guardian, and when does it act?

Question

I'm unclear on several details of how a Shield Guardian works. (MM p. 271)

It is bound to an amulet wearer:

Bound: The shield guardian is magically bound to an amulet. As long as the guardian and its amulet are on the same plane of existence, the amulet's wearer can telepathically call the guardian to travel to it, and the guardian knows the distance and direction to the amulet. If the guardian is within 60 feet of the amulet's wearer, half of any damage the wearer takes (rounded up) is transferred to the guardian.

From the description (outside the stat block):

A shield guardian's solitary focus is to protect the amulet's wearer. The amulet's wearer can command the guardian to attack its enemies or to guard the wielder against attack. If an attack threatens to injure the wearer, the construct can magically absorb the blow, even at a distance.

And it can store a spell:

To do so, the wearer must cast the spell on the guardian. The spell has no effect but is stored within the guardian. When commanded to do so by the wearer or when a situation arises that was predefined by the spellcaster, the guardian casts the stored spell with any parameters set by the original caster, requiring no components.

My questions are:

1. Does the amulet wearer have to use a bonus action or anything to command the guardian?

2. If the amulet wearer does not issue any explicit commands, does the guardian act independently with the general goal of protecting the wearer, or does it act only as commanded by its amulet wearer?

3. Does the guardian act on its own initiative count?

4. The text about storing spells states that the spell will be cast by the guardian with the same parameters as set by the original caster. Do we assume that the amulet wearer can choose a different target for the spell at the time it is cast by the guardian?

5. If the guardian casts "when commanded" -- does the guardian cast the spell on the owner's turn when commanded, or on it's own next turn? Especially if the former, does commanding it to cast a spell use any action on the part of the amulet wearer?

6. The guardian can also cast when a predefined situation arises -- does that mean the guardian can cast as a reaction when the situation arises? Or does the situation have to be something like "if on your turn...." ?

Answer 1

The Shield Guardian is easier to understand if one doesn't go looking for details that aren't there, and if one notes that it's not a mindless construct. Treat it like a normal creature, with only the alterations made explicit in its description, and a lot of creature defaults will answer these points.

1. Commanding the Shield Guardian doesn't require a bonus action, because it doesn't say it does. To command the Shield Guardian requires nothing other than simply commanding it in the everyday sense of the word: saying words to tell it what to do. (Note in particular that its languages entry is “understands commands given in any language but can't speak”.)

   It may be tempting to look at the Ranger's rules for commanding their animal companion, or similar examples where explicit actions are required: this is a trap, because the Guardian doesn't have any such wording. The actual normal case to compare this to is when monster A shouts commands to monster B, which doesn't have the limit of requiring a kind of action to do – they just do it whenever.

2. It doesn't say it can only act as commanded, and it does say that it has a focus on protecting the its master. It also has an Int of 7, so it's capable of simple reasoning at least, and probably more (7 isn't that far below average). Being unable to act unless commanded would be the unusual case that would need explicit statements, so default to normal creature handling: it does stuff on its own, following its own motivations (i.e., its single-minded focus on being a bodyguard), and also follows commands it hears from its master.

3. It acts on its own initiative count, because nothing indicates special handling regarding initiative. It has the capacity to make its own decisions (within its very narrow motives) absent commands, so it's not reliant on the amulet wearer's commands.

4. It doesn't say the stored spell is cast by the Guardian with “the same parameters”, it says “with any parameters set by the original caster”. Setting parameters is not required, then, it's at the original caster's discretion. This gives the original caster some optional extra control over the spell. The rest of the spell's parameters are up to the Shield Guardian when it casts it using normal casting procedures.

5. Commanding is done via words, not a special action that defines how the timing works. Words can be spoken during the amulet wearer's turn (or if your group has loosened the “on your turn” restriction, at any time), so the amulet wearer can issue commands any time they're able to speak. Either way, the Guardian will take action to follow its commands only when it takes its turn, as normal, since no extra wording indicates any deviation from the norm.

6. The Guardian isn't given a Bonus action or a Reaction by the Spell Storing trait, so it doesn't have one to use for this. It just does it as soon as it can, like any other order it follows or action it chooses itself. (It could use a Ready action to set up a Reaction with which to cast the spell, but that would just make it take longer to act after the situation arises, so it won't do that unless the situation is unusual in that the Guardian would get some warning and time to prepare.)