The person with whom I work most closely (she essentially does the same job as me, but is more junior) talks for Britain. Really talks for Britain. One day recently, I lost a two-hour window completely to her talking almost non-stop, and then felt depleted for the rest of the day and unable to knuckle down. And last week, I logged her interruptions just to capture the extent of the problem: 18 in a two-hour period. 
Our jobs centre on research and proposal writing – they're not customer-facing – so it's all heads-down kind of work, and her endless chatter is therefore a nightmare; getting into any kind of flow with all her interruptions is really difficult.
Our manager is aware of the problem and has tried twice to talk gently, in a read-between-the-lines kind of way, to my colleague about the need for her to get her head down, focus, etc. The message has been completely lost on her.
Our manager has now asked if I will say something more direct to her myself, since the problem affects me most, and I've said I will.
I'd just be grateful for some advice on how to do this in a reasonable, effective way, since I'm not her (or anyone's) manager. She's young and fairly inexperienced and I think likes to think out loud/sound me out on everything - everything - for reassurance, more than she needs to. She's also quite sensitive and prone to tears so I know she's going to cry, however nice I am. And I do want to be nice, whilst being honest and firm, because I have to work so closely with her and don't want to make an enemy. And she is a fundamentally nice, sweet person; there's no malice there – just endless wittering!
But my goodness, I need to be able to get more work done! I need most of my energy to be going on the job I'm actually paid to do, not on listening/responding to her.
Many thanks for any tips. (And please don't suggest headphones – they're banned! [weep])