When he began to speak to her he was so serious, she looked at him in confusion. Wasn't an assumption? Ohno ohno ohno. She had time to react, to push him away before he could cover her mouth with his, but she found she couldn't move. And when his mouth met hers she couldn't stop the immediate natural reaction, which was to kiss him back. For a few seconds there was only his mouth on hers and when their tongues met she had the thought that she could easily lose all reason to his mouth.
And that's what woke her up. She pulled back from him and placed her hands on his chest.
"Booth, what are you doing?!" she asked, looking at him with more fear than confusion on her face. She, of course knew full well what he was doing, but needed a second to gather her thoughts.