Have you found a boar drained of blood yet? If not, then look out for one.
The Astarion bite scene appears after you find and click on a drained boar somewhere. I think its location varies depending on where you go first (it did in EA at least) but generally en route to the ruined village (Moonhaven). I generally find it just before the bridge that is on the shortest route to there from the druid grove.
