MacRumors acquired training documents for Apple Store employees showing how the in-store purchasing process will go down. The document reads that customers will need to make appointments to purchase an Apple Watch in-store. If a customer were to just walk in, an Apple employee would offer to let him or her try on a watch and help make an appointment to purchase one.
Remember that this only goes for in-store sales. Folks pre-ordering the device will get it by mail without the hassle of making an appointment. Apple seems to want one-on-one contact with people to help them get familiar with the device. It also looks like Apple will have low stock of the Watch during the first wave and will need to moderate sales. Either way, pre-ordering seems like a better idea.