This is one of the reasons why the game has such a high caliber of game play. You are never at rest. One of the problems with old style games was tha t the monsters were always ahead of you, simply waiting around until you reached that area. Half Life uses an interesting twist on this with varied enemy locations. I was walking down a dark corridor with my flashlight in hand while trying to peer deep into the darkness, when suddenly an alien popped up behind and to the left of me to zap me.
Half life has other twists on the genre as well.
There were cave-ins with aliens falling from the ceiling. Then there were the 250 feet monsters that came through a door and snatched me into their mouths. The game sports a very action packed package, with intense battles. Better duck or that bolt of electricity will zap you right out of existence. This then leads us to the mother of them all, Artificial Intelligence.