I'll make you beg for more…
Dean knew that the demon couldn't be trusted, of course, but it was human nature to try and get a deal out of another person, hoping it would stick to it. That was why he had talked to it, telling it what would happen if it didn't stay put, not because he thought it would stick around if his attention wavered. The lines on the woman's face sure looked scared though and the demon hadn't tried to escape through the devil's trap yet. Perhaps it would be possible to bring it in.
The first miles he kept his eyes locked on hers, wanting to stare that blackness down, showing them that he knew all there was to know and still could face them, but she had shut her eyes almost immediately. He and Sam had a brief conversation with each other and with Bobby, then they both concentrated on getting there in one piece. Sam by paying attention to the driving and Dean by making sure the demon didn't try to escape.
After a while Dean noticed that the lines of fear in the woman's face had turned to pain and that she was breathing in a way that spoke of her trying to control that pain. At first he hesitated about what to do. Could the host body be taking these steps itself, in trying to preserve life? Or was it a clever ruse from the demon's side, making it look as human as possible? He decided that he couldn't take the risk of watching the woman die in front of him, just because she was controlled by a demon at the moment. If she still lived he would do his best to free her.
He slowly eased the pressure on the torso, allowing her to shift position of her arms. Then he lifted the noose just a little, careful not to let the hand holding the holy water bottle stray far from her face. She didn't open her eyes but her body relaxed infinitesimally and her breathing came easier.
This was way too human, Dean didn't like that the demons were this good at imitating them. He suddenly felt like the bad guy, crippled with guilt and doubt, which probably was this particular demon's attempt at lowering his defences. Still, he stopped pushing and just kept guard. On her movements and his own feelings, keeping both from getting out of hand. You shouldn't feel like a serious bad guy because you had trapped a demon, for heaven's sake. And he fervently hoped that the demon had pushed the real woman far under, he wouldn't want her to remember an experience like this.
"Drive faster, Sam", he mumbled and felt the Impala shoot forward a bit in response.