I have never experienced horrible paranoia on crack, but I have had small bits where I was probably right to be paranoid... this would be when i was smoking crack in my home as a teenager, taking a blast before my shower, smoking in my closet at night because I could not wait until the morning, etc. All these times I would be so scared my parents would come in and find me using, I would take a blast and sit with my ears perked for any stir or noise.
I did use with one woman about five times who was super-paranoid like you explained... always looking out the windows, peepholes, checking locks, not responding to anyone because of whatever she was imagining. I vividly remember her being in like slow motion -- very slowly put a chunk in her pipe, hit it, exhale, all while staring out the window, only glancing down at the drug to make sure of the amount and if it was melted onto the pipe properly. She was a real buzzkill to be around, made me paranoid watching her.
I liked to have fun while smoking. I would make up little games like spinning after taking a hit to make the ringer hit faster, smoking in a bathroom to listen to the tinny sound the fan or running water makes after a hit, sitting face in front of fan after a good hit, feeling the air on your face and distortion of your voice. It's all fun until you realize you are desperate for that next hit -- then you see that it's not so fun anymore and it's scary the pull it has on you .