If you find yourself with nothing to do, ask if you can assist anywhere and make yourself available to help. This will not only show that you are capable of following instructions, but going beyond them to offer extra support.
If there is not much to do, don't just stand around. There are always peaks and troughs during Stints and some moments are much quieter than others. Always try and look busy (even if it means wiping down a table you have already wiped before!).
Standing still with your arms crossed is often a natural stance many of us turn to, but it immediately gives the manager the idea that you are not working hard. It only takes them looking over once to make up their mind!