|Image from here|
Just as "Night Terrors" was the injection of good old Doctor Who monster fun the series needed, "The Girl Who Waited" is the character driven episode that both Karen Gillian and Amy Pond were waiting for.
After the Doctor,Amy and Rory decide to spend some time on 'the second best holiday destination in the universe' (because the first one is too tourist-y) Amy becomes separated from the Doctor and her husband, separated by nothing more than a slight miss-communication about which button to press. It's a small piece of symbolism about married life, one partner doesn't express requirement/instruction clearly enough and the other doesn't ask for clarification. However here, instead of hilarious unintentional comedy moments/domestic arguments breaking out Amy ends up trapped in a compressed time stream - resulting in her being... a tad older... once the Doctor and Rory breaks through into her time stream.
The whole episode is an interesting piece, the Doctor is moved to the sidelines while Amy, Rory and yes, a second Amy take center stage. Karen is given the lion's share of the acting in this episode and with the second, older Amy she has the chance to play someone more experienced, more isolated and bitter about the Doctor. In this she echos many past companions, angry about being left behind by the Doctor. For the role Karen developed new body language; an older, angrier voice and donned some facial prosthesis to make her look older - her appearance is a little off, but her physical performance and acting sell the character.
"The Girl Who Waited" is one of the high points of the current season, it's not as good as "The Doctor's Wife", however it's much better than most of the other episodes, the 'villains' are interesting (even if at times they look like dancers from a cheap pop video), Karen and Arthur are both given a lion's share of acting to carry, which develops and deepens the relationship between Amy and Rory, and Amy is shown to have hidden depths beyond the norm - able to develop/design a
sonic probe (screwdriver) and survive for forty years alone against constant enemies without gaining a single grey hair.
If I had one critique, it would be that old Amy should have been released in some fashion, to allow Karen the chance to return to the show twenty/thirty years from now and play the part of the freshly freed Amy. But this is a minor quibble in an episode that was mostly gold.