Bruce stays relatively tense while Clark kisses him, even growls a little as he bites and chews back, in spite of his very obvious arousal and physical response. Not only being turned on but pushing away from the bookcase to arch against Clark, grind into him even.

The point that changes - the tension, the wariness, the caution that comes with a threat that isn't nearly well enough known, and all the scheming and planning that his brain insists upon doing in the face of that - is the second Clark ducks down toward his jaw and doesn't go for his throat.

At that moment something in his muscles and center of gravity changes and he slips one hand up into Clark's hair and the other against the small of his back. Arches again, more fluidly and less violently - but still somewhat... watchful in demenaor. Testing, perhaps.