When Shepard first approaches, he tenses, if only imperceptibly; draws in enough to let him retaliate if an attack is what's coming. When she kisses him instead, he stiffens, arching his back in surprise, if only for a moment.

Then he doesn't.

He'd been playing, before, flirting with Shepard. He's undoubtedly attracted to her, but he's attracted to a lot of people, and flirting with them is just part of his personality: he never expects it to go anywhere. If it does, it's at their discretion.

And Shepard's discretion, apparently, is this.

There's only a split second of hesitation before he leans into her; he shivers when her teeth scrape his lip, and his hand curls against her armor, not shoving her away but looking for purchase. She presses him against the bulkhead and his only impulse is to press back.

Then she pulls back, and she leaves. Lavellan hardly hears what she says to him, still reeling from what just happened. He just sits there and watches her go, boneless, his hair and his clothes totally disheveled, the surface of his skin almost uncomfortably hot.

That's not at all what he expected to happen. But, also: wow.

Boner alert.