There's nothing like thinking you're prepared for something and...finding you are, in River's case, which is to say that she gives a blink of consideration to the hot pain when Charlie makes contact before she grabs his arm in return, yanks him close and headbutts him as hard as she can before swinging him with all of her might backwards away from her into the wall furthest from the window.

Of course his hand leaves an impressive hand-shaped burn in her skin but she can deal with that ...later. Now she has a split second to jam her elbow into Charlie's jaw and hope it dazes him enough.