Here's a kind of funny question. Suppose one is interested in some specific set of materials. The music of Morton Feldman. The poetry of Federico García Lorca. Ancient Minoan archeology. How children learn their first language.
It cannot be a question of writing a left-hand column of theoretical approaches and a right hand column of research materials, and then just finding a good match. First of all, there will be a methodology already entrenched in a field. Archeologists already do things in a certain way. for example. Secondly, the kind of research problems one is interested in are inherent to the materials, in some sense. If the wrong theoretical questions are asked, then the original source interest of the materials can fade away.
If you haven't asked the theoretical questions yet, it is as though you hadn't thought about your materials as an intellectual. If you don't have a theoretical approach that is your own, that is part of your own intellectual identity, then you will just be applying a theory because a professor told you too. That is perfectly fine for a Graduate Seminar, because that is a purely academic exercise. It is harder as a professional scholar to use a theory you don't really believe in.
You can use theory as a heuristic, or as part of your intellectual identity. In the first case, you are not professing literal belief in the theory, but using it because you think it usefully illuminates the text. In the second case, there is more of an exisential commitment.