Faye wasn't sure which drove her more: the knowledge that Spike wanted to keep the girl - or, as she assumed, young woman - oblivious or the appeal of River's insistence. Perhaps even the wording, which she believed to be deliberate, played a role in the choice. Whatever the case, it hadn't taken much cajoling to convince her to... share the vaccine.
Why shouldn't she? If Spike cared so much about the girl, why would he choose to doom her to certain death? Granted, Faye personally didn't believe that their captors would break pattern and unleash such a plague where few people stood a chance of survival; but if she happened to be wrong, the window of time between infection and survival would depend completely upon method of release. There was no guarantee that the beautiful illusions would come slowly, no promise that the heads wouldn't attempt to recreate Vincent's doomsday plans on Halloween - should they somehow decide to go that route.
She just didn't understand why he would play such a cruel game with his allies. It made no sense at all. He wanted to warn them, but couldn't think to warn of the beginning symptoms... It was ignorant. Stupid. Another reason to defy his ridiculous scheme to keep River away from the details.
One of many reasons, as it stood. Faye couldn't convince herself that there wasn't a significant dose of selfish whim involved. Curiosity, intrigue...
When she heard the knocking, Faye chuckled lowly and shook her head. It was easy to assume that River had started the venture before being told that the door would be unlocked, she supposed. But from what she'd seen of the girl so far, she also thought it might be a strange quirk. Heaven knew River had her fair share of those.
"You can come in, River," she called out, her voice taking on the sultry sort of tone she reserved for such situations. Or the sort of situation she expected this to be. She adjusted her position on the bed's edge slightly, crossing her legs and turning enough to see the door. Provided the girl actually entered when invited, she greeted River with a soft smile, slightly bouncing her topmost leg.