While Zombicide is all fun and what not, there isn't much of a fear factor. "Sonic Bunny" over on the Board Game Geek Forums (BBGF) came up with this awesome idea for the game.
When spawning zombies for any reason, instead of placing physical models down, only place the spawn cards on the board. Each zombie turn move the cards as if they were walkers. Do not reveal the type and number of zombies until a survivor can draw line of sight to the spawn card. When any survivor gains line of sight on a spawn card, reveal it and place zombies as usual for the CURRENT experience level (blue, yellow, orange or red)
Sounds pretty cool... and you don't know what's coming until you get to them. This can be applied to zombies inside of buildings as well as the ones on the street.