So, this mannequin challenge thing is really popular, huh? Well, now you can do it Left 4 Dead 2! Step 1: Launch the game (really, me, really?). Step 2: Type in the developer console "sv_cheats 1". Once done with that, load up a map: ex; c1m1_hotel. Then open the console once the map is loaded and type in this command "nb_stop 1". Boom! You are done! All animation will be frozen, even facial animations (unless a survivor is talking)! Here is the result:

Not all things freeze. Animated textures still, uh, animate. Particles, like the Smoker's smoke don't stay. That's about it in terms of downsides. Now go out there and make a Tank do the mannequin challenge!