Because you are an artist and the theater is there to promote art and culture, you and the theater administrators are partners. You need to show that you are reliable and dependable-zuverlässig is the exact German word, and that you can have a good dialogue with the administration. This is often known as being a “gute Gesprächspartner”. More than anything else this requires that you understand your rights and obligations as well as the needs and rights of the theater (and the particular administrator) you are dealing with.
Your first step is to read and understand your contract, preferably BEFORE you sign it, and discuss it with the agent or some knowledgable friends or colleagues. Much of it will be standard and formulaic and some will be critical to what is expected of you as an artist but all of it is binding! They are paying you every month and contributing to you health insurance among many other things. The bank will open an account and even give you a line of credit based on your job as “Opernsänger”! But to be a good “Gesprächspartner” you also have to understand what the administrator and theater needs and how you fit in. Then you can have a dialogue to try to work out your needs as a singer, artist and human being, with what the theater or administrator wants or needs. Engaging in this dialogue is critical to getting what you really need and being considered a good member of the ensemble-someone who can be recommended to other theaters and agents and rehired.
It’s very important to understand when you or they have “Recht”, or the right, on your/their side. It is literal and means their is a rule, regulation or contractual ground. You will do well to consult with colleagues or the union rep (GdBA is the union for all theate employees and one of the soloists is probably a rep) to know precisely what your rights are in a situation and administrators will respect you if you know yours. There is give and take and you should know that if they cut you some slack they’ll expect you to help out some other time and vice versa.
You should generally use the respectful formal “Sie” in such discussions. If they address you as “du” it may be taken as an invitation for you to respond in kind, a sign that you are colleagues and working together.