Imposter will not activate if the opponent is transformed, disguised with Illusion, or behind a substitute. Imposter only activates when the Pokémon with the Ability is sent out; if it fails to transform into an opponent, it will not activate unless the Pokémon with Imposter is withdrawn and sent out again.
A transformed Pokémon can use moves exclusive to a species it has transformed into, such as Hyperspace Fury if transformed into Hoopa Unbound. A transformed Pokémon can use Z-Moves exclusive to a species it has transformed into, if it holds an appropriate Z-Crystal. If the Pokémon with this Ability has transformed into a Mega-Evolved Pokémon, it cannot use any Z-Moves, even if it holds an appropriate Z-Crystal.
If the Pokémon with this Ability transforms into a Pokémon that has no type (i.e. it was a pure Fire type that used Burn Up), the transformed Pokémon will become Normal-type; if such a Pokémon has additionally been affected by Forest's Curse or Trick-or-Treat, the user will become Normal-type with an added Grass or Ghost type respectively. If the Pokémon with this Ability transforms into a dual-typed Pokémon that has used Burn Up, however, Imposter will copy the user's current types (and type additions) as usual.