If you're the sous and you screw up, the (good) exec already has a backup plan. If you're the exec and you screw up, there is no backup plan. If you're the sous and can't come up with an idea, you can talk to the exec. If you're the exec and can't come up any ideas, you look silly. If you're the sous, "food cost" is only something that you're mildly aware. If you're the exec, "food cost" is something you can recite better than the pledge of allegience.
Seriously though, the basic thing is that the sous acts as the exec whenever exec isn't around, be it that he's off that day, in another part of the kitchen, whatever. A good relationship with similar culinary outlooks and open lines of communication between an exec and a sous will make the restaurant as a whole that much better. If you're a sous, don't try to undermine the exec about anything when he's not around, instead discuss it behind closed doors and try to reach the best solution to any problem. If you're an exec, make sure you pick a sous that you can work with easily and whom you can trust.
The first time I was a sous, I was mostly ready. It's likely because the job didn't entail much more than being a quality line cook and teaching others how to be the same with some mild ordering duties. The exec slowly gave me more responsibility and gave advice about how to handle it. He eased me into the position. I had never been in that spot before, and he made me comfortable with it. I'll never thank him enough for it. The first time I became exec was when the formerly mentioned exec moved a few hundred miles away. That was mostly sink or swim. Thankfully, he had taught me a lot and I didn't do too bad. The first month I was doing over 65 hours a week trying to get everything together, but eventually cut it to about 50. Efficiency is key, timing of when to do certain things is key, trust in your new sous is key. I picked up the first two quickly, but the last one was trouble. The owner picked my sous for me, and while he was a great cook, he was never able to fully grasp the more managerial aspects of it. Such is life.
Ready to become a sous depends on the restaurants. Volume will be a big part of how much responsibility you have. A restaurant that seats 40 will be a lot easier than one that seats 100, even more so than the numbers indicate. Just be able to cook, able to motivate, and able to correct mistakes in a way that people will learn and not feel resentful.
On the other hand, I don't think anything prepares you to be an exec other than decades of experience. Strong math and statistics ability will help, as well as the abilities for a sous. It's tough to keep your employees happy and motivated, satisfy the owner, gauge your pars for each individual item, create good specials, all while spending at least some quality time on the line. It's like you're forced to juggle when you never learned to juggle, but you'll eventually pick it up. After a while, it's second nature that you anticipate problems before the happen, and your staff will be in awe of you. That only helps to assert your authority in a nonconfrontational way, which is always good.