Doctor Who Magazine #495

River has to interact with the Eighth Doctor to save him, to help him. When she’s figured out how she can do that without having to be physically present in front of him, alongside helping him, she can then tease and flirt a little bit, because she knows she’s safe, and she knows she hasn’t overstepped a boundary, or changed his or her future in any way by that actual physical interaction. So she can then have fun! She’s in control.

What the Doctor says to River when he realises her acumen, her calibre, is ‘Wow, hang on a minute – where did you pick up all this?! For an amateur, you ain’t half clever'. He says, ‘Any time you fancy joining me, I could always do with a pair of hands’. He realises, almost instantly, how clever she is. ‘If you’re ever stuck for a trip round the universe helping me out, you’re on.’ So there’s that lovely playfulness as well. Of course, he’s doing it in blissful ignorance. And she’s trying not to appear too clever, and give the game away, but of course she has to save his life. So it’s great. It’s childish, but it’s great fun.