Summarize:

A spinal fracture (also called a "vertebral fracture") is a very serious injury that can cause permanent damage to the body, though this is not necessarily guaranteed. Spinal fractures occur when one or  more of the vertebrae in the back crack or come apart. The most immediate symptom is intense pain in the middle or lower back that makes it very difficult or impossible to continue doing what you're doing. This pain may be similar to what you might get from breaking another bone in your body (like, for instance, an arm bone), only localized in the back. Faking this sort of pain can be a challenge. When the "injury" occurs, you'll want to cry out in pain, fall to the ground, and immediately start wincing or writhing in intense pain. If you can cry on command, this is a good time to use your skill. Like most broken bones, spinal fractures lead to persistent pain that lasts long after the initial injury. This pain is especially bad when doing anything that puts even minor stress on the back. This includes:  Standing Walking Getting up or sitting down Bending Twisting One of the worst things about spinal fractures is that even lying in bed does not reduce the pain completely. Since it is impossible to lay horizontal without putting a little stress on some part of the back, even bed rest will be painful, though not as much as standing or moving around. Typically, for real fractures, this is mitigated with pain medication and narcotics. Because a spinal fracture results in actual physical damage to the bone structure of the back, it can cause noticeable changes in a person's stance and posture (though this was more common in the past when treatments for this type of injury were limited.) You may want to imitate this sort of damage as part of your act. Specifically, spinal fractures can cause:  A "hunchback" appearance A decrease in height The inability to stand straight up When a spinal fracture occurs, the bone from the broken vertebra can press against the nerves of the spinal cord (though this does not always happen.) When this occurs, symptoms that are very similar to those from a pinched nerve caused by a herniated disc can occur. These include:  Shooting pains in one or more limbs A numb, tingling sensation like the limb has fallen asleep Weakness and stiffness in the affected limbs In severe cases, a loss of bladder/bowel control Spinal fractures are usually caused by sudden, violent injuries. It can be difficult to seriously fake these sorts of injuries — for instance, convincing your friends that you've just been in a car crash will require lots of work. However, knowing the sorts of injuries that can cause spinal fractures can still be helpful if you're pretending that you had one long ago. Examples of injuries that can cause spinal fractures include:  Serious car crashes Long falls Gunshots Violent sports injuries (tackles, etc.) Combat injuries Note that all of the examples above will usually also give you other injuries like broken bones, scars, lacerations, and so on. If you are aiming for authenticity, be aware of this. A spinal fracture is not something that can be treated in the home with a few over-the-counter pills. Spinal fractures require immediate medical attention to assess the damage done, administer drugs, and start addressing treating serious problems like nerve damage and so on. Don't seek actual treatment for your fake spinal fracture. As mentioned above, this is a blatant misuse of medical resources that can have major consequences. However, if you're pretending that you've already been dismissed from the hospital, you may want to do the following:  Wear a back brace or a spinal cast Stay off of your feet Wear compression stockings (special socks designed to keep blood clots from forming in the legs for people who are bedridden) Imitate the nerve damage symptoms indicated above Take small doses of over-the-counter medications for pain and inflammation. Don't take actual medical-grade painkillers, as these can have serious side effects if misused.
Pretend to have severe, debilitating pain in your back. Act like the pain "flares up" when standing or moving. Keep faking moderate-level pain while lying down. Keep your back curved or hunched. Optionally, fake nerve damage. Have a good story ready. Pretend to be undergoing treatment.