There are tricks you can do to draw attention to things. The Portal and Half-Life 2 Episodes developer commentaries include quite a few discussions of how they

There are tricks you can do to draw attention to things. The Portal and Half-Life 2 Episodes developer commentaries include quite a few discussions of how they designed their environments to nudge players into noticing stuff. For example, in HL2 Episode 1 there's a scene where you're walking across a bridge and a huge helicopter comes up beside you. To ensure that players were looking the right way to be surprised by it, they put an enemy in the distance firing (ineffectually) at the player to attract attention.