With stalling, make some parts, more confusing, make puzzles of a sort, have the player have to think, or give it somthing interesting to make it more difficult, maybe make then enemies scarry in a sort, if there human give them big weapons with big power to scare the crap out of the player, sounds can make thing much more intense, also make things bigger, but don't apply alot of details, if you have overlays, throw them onto a special segemnts thats the same as the original, but with the overlay to help with resoureces. If you have papers or keys on a desk, draw it in to make it look 3d to the table texture, but save the origiinal and script the texture change and have the one with no papers or keys or whatever on it. I belive there's some way to get static models to run scrips, somehow, or you could have it dynamic, static seems to screw up the level. Also an interesting way to make ur enemys seem intimidating is to attach a double sided plane to it in the modeling and give it an animated texture (i think there's a shader for that) to show the firing and the flying bullets. By making enemys more intimidating, you make the player take caution, which makes them take there time.
