When a character casts a spell, a number of decisions need to be made. Some choices are explicitly made by the player. For example, a character can choose the target of a spell (emphasis mine):
A typical spell requires you to pick one or more targets to be affected by the spell’s magic. A spell’s description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect. (PHB)
Certain spells, like Control Water, explicitly allow a character to make additional choices.
Until the spell ends, you control any freestanding water inside an area you choose that is a cube up to 100 feet on a side. You can choose from any of the following effects when you cast this spell. As an action on your turn, you can repeat the same effect or choose a different one…
Other spell, like Earthquake, let the GM make certain decisions:
Fissures. Fissures open throughout the spell’s area at the start of your next turn after you cast the spell. A total of 1d6 such fissures open in locations chosen by the GM. Each is 1d10 × 10 feet deep, 10 feet wide, and extends from one edge of the spell’s area to the opposite side. A creature standing on a spot where a fissure opens must succeed on a Dexterity saving throw or fall in. A creature that successfully saves moves with the fissure’s edge as it opens.
Still others, like Conjure Animals (who decides the exact creatures), Major Image (who decides the exact appearance of the image), or Misty Step (who decides the exact destination), leave the decider ambiguous.
In the case of summoning creatures, the Sage Advice Compendium gives an official interpretation, under the question “When you cast a spell like conjure woodland beings, does the spellcaster or the DM choose the creatures that are conjured?”:
Other spells of this sort (summoning spells) let the spellcaster choose from among several broad options. For example, conjure minor elementals offers four options. Here are the first two:
• One elemental of challenge rating 2 or lower
• Two elementals of challenge rating 1 or lower
The design intent for options like these is that the spellcaster chooses one of them, and then the DM decides what creatures appear that fit the chosen option…
A spellcaster can certainly express a preference for what creatures shows up, but it’s up to the DM to determine if they do. The DM will often choose creatures that are appropriate for the campaign and that will be fun to introduce in a scene.
Does this interpretation extend to other ambiguous spells like Major Image or even Misty Step? When a spell requires a choice, and the decider is left ambiguous, is that choice ultimately made by the DM? If true, I realize that in most situations, a reasonable DM would enact the preferences of the spellcaster, barring certain abuses. However, I’m curious about what the Rules as Written have to say about this.