The only really external factors in music are the printed indications, and even these are far from absolute. Dont trust the score too much - to be REAL precise you need to know more than that - plus reprinted score is inacurate - see if you can get the copy of the original handwritings.
Take, for instance, the famous Chopin rubati . The tempo indication is rubato - but how much? What shall the actual notedurations be? To approximate anything resembling accuracy, one needs a sound knowledge of Chopin's life, his moods; what he was experiencing and feeling when he wrote that particular work; its relation to his work as a whole, etc. One tries to reconstruct all this, and then to apply it. Interpretation does not mean borrowing "effects" to round out the rendition of a given work. It means learning how to think musically in approaching all works.

This requires preparation.