Do what the teacher asks, if you are unclear, ask again, so that the teacher doesnt have to redo what you have done
When there is a quiet time, find out if there are any odd jobs that need doing - sticking workpages in books, tidying up book corners, toys, etc., Establish what time consuming jobs need doing on a regular basis that you can do ad-hoc whilst waiting to get on with your main task.
Dont just sit around.
If you notice anything with a child, make notes and talk to teacher at the end.
Never ever ever discuss what you see, hear, think, about any child in the class, no matter how friendly you might be with another person. What you see in class stays in class.
Never discuss comparisons or comment where your or someone elses child is in the pecking order
If reading books, ask the child probing questions about what they have read to ensure they have understood it, can they discuss it. Use the books for more than just reading words. Encourage enthusiasm, expression, flow, etc. If you feel a book is too hard/easy, dont say that to the child, talk to the teacher, but probably best after listening to several books at same level. Some books can be easy in a level and others hard.
Dont shout at kids. Dont discipline them unless you have been given the authority.
Make sure you follow teacher's rules if helping with writing, ie not correcting spelling mistakes etc