For actors this means having lines memorized but it also requires other skills from an actors toolbox, all the years they've spent studying the craft and the experience of being in shows.
For someone working backstage being ready means always having the tools you need to do the job. For technicians this means a multi-tool plier and a flashlight. For wardrobe people this means some sewing supplies, safety pins and a flashlight. Being caught without these things slows the process and it might be the difference between making a quick repair or not.
For writers I think we always need to be prepared for inspiration and ideas. Our tools are pens papers and of course books. You don't want to be stuck without a pen when inspiration strikes. My college roommate carries around one of these all the time and uses it to write down quotes and ideas.
I buy those small notebooks from the dollar section at the craft store like they are going out of style. I keep notecards with me. I try to always have a book with me so if nothing else I can read while I'm waiting in line at the DMV.
What writing tools are you never without?