If Wall of Stolen Identity copies a creature that’s normally not a creature (such as Mutavault), the copied permanent will be tapped and will remain tapped even when it’s no longer a creature for as long as you control Wall of Stolen Identity.

If the copied creature has in its mana cost, X is considered to be 0.

Any enters-the-battlefield abilities of the copied creature will trigger when Wall of Stolen Identity enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

If Wall of Stolen Identity somehow enters the battlefield at the same time as another creature, it can’t become a copy of that creature. You may choose only a creature that’s already on the battlefield.

If the chosen creature is a token, Wall of Stolen Identity copies the original characteristics of that token as stated by the effect that created the token. Wall of Stolen Identity doesn’t become a token in this case.

Once Wall of Stolen Identity has entered the battlefield and stolen a creature’s identity, players can take actions before the creature is tapped.

If Wall of Stolen Identity leaves the battlefield before its triggered ability has resolved, the copied creature will be tapped, but it will untap as normal.

Wall of Stolen Identity can copy a tapped creature. That creature just remains tapped and doesn’t untap during its controller’s next untap step for as long as you control Wall of Stolen Identity.

If another player gains control of Wall of Stolen Identity, its effect keeping the creature tapped expires. It won’t keep the creature from untapping anymore, even if you later regain control of Wall of Stolen Identity.

Wall of Stolen Identity copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If the chosen creature is copying something else (for example, if the chosen creature is another Wall of Stolen Identity), then Wall of Stolen Identity enters the battlefield as whatever the chosen creature copied.