It depends on who asked you.

If you were asked by the main photographer, she's your boss. Generally, you do what she asks you to do, and if you have your own ideas, you may express them, but she has the final word - because she has taken the responsibility for recording the event. If she asks you to assist her, so be it - you have the common goal of delivering good photos. Bringing an assistant and second shooter would stretch the budget too far for most couples.

If you were asked by bride/groom, the main photographer is more like a colleague for you - you may discuss ideas with her, but since you have your own promises to keep, you have full freedom of choice as long as you don't get in her way. In this case, it's best to discuss together with the hosts and lead photographer what is expected from each photographer. These agreements may include assisting each other for some critical shots - your common goal is still to ultimately provide memories recorded in the nicest way.

Pay should be negotiated with the person who asked you, and should be such that it feels fair compared to your responsibilities (including how much time you will be expecting to spend during and off the wedding) and how the images will be licensed. So it's a good idea to discuss the pay together with responsibilities and licensing.

When you are the main photographer, you have your own goals for which you bring the other person, and these goals will be basis for your agreements with him/her.