They will typically arrive a couple of hours before you and complete the snag on their own, taking their time to capture everything. They would then normally ask you to meet at the property and walk you through everything they have spotted and take note of anything else you want included in the snag.
It's also worth including items that are not necessarily snags but should be completed, e.g. your contract might state something like electric fireplace installed. Your surveyor might just think you will be installing it yourself so they wouldn't necessarily include it. The same for all other fitted units in kitchen and bedrooms. Make sure your surveyor is aware of the finish you expect and what is in the contract