Matt Smith has said on BBC Radio 1 that the identity of River Song will not be revealed until the second half of the current series, which will return in the autumn this year. However, he insists that her true identity will have "serious" consequences for The Doctor.

Smith said: “It does develop nicely and there are huge cliff-hangers throughout the coming series; there's a great one at the end of episode seven which is a killer and at the start of episode eight we really do learn who she is.”

Matt relishes the scenes he gets to act out with Alex when The Doctor and River meet, as they did in the opening two episodes, and he thinks the time-traveller is one of the few people in the 'Doctor Who' universe who can challenge the Time Lord.

He added to BBC Radio 1: "Well the first time The Doctor met her she died and then she's come back under the tenure of Mr Steven Moffat and we don't know what she is. She just kinds of swoops in and swoops out and causes all kinds of madness and havoc and saves his life on most occasions.

But she knows more about his future than he does which for The Doctor is a very strange and bizarre concept because he usually knows the most and therefore it hopefully creates for quite a funny and flirtatious relationship. It makes The Doctor uncomfortable, which I think is good because not a lot does."

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