The way in which Half-Life maintained a continuous viewpoint over long stretches of gameplay and landscape was always so immersive to me. Games like God of War and Dead Space did something similar, but Valve had an additional challenge.

They almost never take player control, instead relying on mere hints of where to loo; they even have the character sequences scripted for wherever the player was standing. That all usually took a lot of their effort.

I could be biased because I even enjoyed toying with their choreography tool, which let you layer simple gestures together; so without making a new animation, you could have someone both lean forward and nod right, and point their thumb right.