Dear Bob:
OK, I got your point now. There is one answer: you must know in depth each language, each style, each composer. Because the question is not whether you can realize the interpretation of a piece, but if you can control your options in the given limits.
One example: when I work a new piece by Mozart, I have a very good idea about what to do at first sight. Why? Because he is a composer that I am very deep into, and not only his piano music: his chamber works, symphonic, vocal, operatic and so on (what is sometimes neglected by some students/performers).
In the other hand, if I start a piece by Mr. AlphaBeta, I need more time to get acquainted with what he is doing, and probably would be highly welcome some background research (other pieces and influences throughout) and a complete analysis of the score to have a proper notion.
(It's important to tell you that I think that free invention, inspiration, imagination, "mood", and the like, are framed and controled by style and historical knowledge about it. Taken that for granted, you must consider how much is important to create an interpretation without elements to do it. Yes, I know that a lot of performers do it, but they are normally known for the fact that they play anything, from Bach to Liszt, in the very same way. I recognize that works sometimes, but now-a-days it's not a serious option anymore.)
Finally, let me write some words of warning. Some of the best insights and most of the worst mistakes are product of the first contacts with a new work. Thus, you must keep your mind open. Interpretation is something that grows naturally, even when you work with a very well known style, composer and, for that matter, piece. It seems like a contradiction, but it's not: even when you are able to have a very good idea of the interpretation at glance, it will develop through time.
Now, there is a problem (that is the topic of another thread up here): it's easy to loose the freshness or intuition after a couple of months of daily practice. My antidote is simple, and I wrote it before: keep your mind open and try always to surprise yourself. Furthermore, it is near impossible that anything above average that you play will be 100% ready in the first try. Normally, you start and keep working for five or six months. Then, it's good to quit. A year after, resume: it will be much better. Play again for a couple of months; quit again. And so on. Some pieces take one cycle, other two, and another you carry through your life until you have your perfect rendition done.
What do you think about this?
Best wishes!