I don't think it's that unusual. I recall listening to the neighbour's kids, before I had my own, acting out scenes from their favourite movies and correcting each other if they got the lines wrong ("My name is Inigo Montoya. You killed my father. Prepare to die!!"). It was very important that they get it perfect - right down to the pauses in action and speech. I think there is a certain developmental aspect to mimicry. It's probably hard-wired to a certain extent as a learning mechanism.
It is frustrating if that's the only kind of play they will engage in. As contradictory as it sounds, perhaps you can lead them into more open-ended play. Acting exercises are terrific for this sort of thing. Some of DD's favourites (she's a drama major):
- Charades - good ol' fashioned game that most kids love to play
- In the Manner of the Word - one player (the guesser) leaves the room, and the others choose an adverb (sadly, proudly, happily, sweetly....). When the guesser returns, s/he can ask the others to act out a task (shopping, dancing, eating dinner...) in the manner of the word. The others act it out in silence until the guesser has figured out the word.
- Defend/Attack - (Only if you don't mind a little conflict!!) Sort of a "rock, paper, scissors" game, only you use your whole body. Each person chooses a defence and an attack that they can act out when called upon. Two people oppose each other - one is the attacker and the other is the defender. For the record, "mind attack" and "impenetrable shield" are not great choices - pretty boring, in fact!
There are lots of improv games, if you google or check the library for resources on theatre for kids.