There's an almost painful tension in his shoulders and jaw. Clearly he's worried about hurting Cloud in so genuine a way that it actually makes him nervous. The visible relaxing of his broad shoulders into a relieved slump is proof enough of that when Cloud reassures him.

Zack nods, more for himself than Cloud, and finds himself a little brighter for it. So with his free hand, he pops the cap open, and manages to get a good amount on his fingers. Thankfully, Cloud is small (comparatively), so handling him a bit to get between his legs isn't difficult. It does have Cloud mostly held against Zack's chest though, Zack still lazily pumping them both.

"Probably gonna feel a little weird at first, but don't worry. I'm just gonna start with one finger so it shouldn't really hurt. You ready, buddy?"