When a part of a movie/show's soundtrack happens along with something on screen (e.g. an orchestral sting punctuates a character's glance) how are they synced up perfectly?

Is the final cut of the visuals produced, and then the soundtrack recorded to match that timing, or are the important moments in the soundtrack synced in editing? I can't imagine it would make much sense to try to get actors to time their actions with the score.