"You cannot give what you have not got" - Horace
Our interpretation of a piece depends upon our level of understanding about the piece. Understanding comes from study.
When I was in school, most of my theory courses focused on the harmonic structure of music. This kind of analysis is important, but even more important is to look at a score simply as a graphic image.
If we think of a map, we understand that it's not the territory itself, but it represents what the territory is made of - what roads intersect, what changes there are in the land, what landmarks there are, etc. We get a deeper understanding of a territory by looking at its map.
In music study, I encourage students to look at their pieces like maps to collect facts about the music. The more we can notice on the map (or music score), the more we understand the territory (or music).
Here are some ideas to think about when looking at your music. Do this without the guitar in a relaxed, interested state of mind. Write any ideas that pop into your mind onto the score.
1. Images - What does this piece represent to your imagination. If it were a film score, what would it be? A king entering the court? A horror film? A mother singing to her child? How would that image "feel" musically? Do the scenes change? What about the camera angles? Let your imagination be free and write your ideas in the score.
2. Texture - is the music very thick (like 4 part harmony, or full chords) or is it thin (maybe a single line or two lines)? Notice that the textures change throughout the piece. How will you express those changes?
3. Accidentals - especially in Tonal Music. Accidentals are signs that something has changed in the tonality. We want to feel those changes and express them.
4. Lines - where does a melody begin and end? What about the Bass? What about the middle? Do any lines have something of interest? Do the lines jump around or are they linear? How would your playing express those differences? Many times you will notice that when one voice ends a melody, another voice then adds something of interest so that there is no "dead space" in the music. Pay attention to those "bridges" as well.
5. Counterpoint - one of my favorites. Look to see how the voices interact. Even in the simplest piece by Sor, the counterpoint is wonderful to witness and enjoy.
6. Rhythm - what is the underlying feel of the piece? Even in a very complicated piece there are basic points in each measure which give a "grounding" to the rhythm. Rarely do all the notes get the same emphasis. Find the groove by singing, tapping, and feeling it in your body.
7. Rests - do you notice the rests? Many times a rest is as important as the notes - or even more so. Notice all the rests in the score and choose if you are going to express them or not. Also notice if you need to stop notes from ringing over each other - for example, in bass lines with many open strings.
8. Articulation - Sing the piece with feeling - do you notice your articulation is different than when you play? How about vibrato? Notice the differences and experiment with them in your playing.
9. Harmonic Analysis - what is happening in the harmony? Are there surprises in the harmony? like I- IV - V - iv. If there are twists (and many times there are) we want to express them.
Other questions to ask may be -
What is the Highest / Lowest Note in the piece? Where are they?
What Dynamics are indicated? Why?
What fingering is used by the editor? Why?
What attracts you to this music? Does it remind you of something or someone?
How would this sound as an orchestra piece? What instruments would play which part?
How would this sound if (insert your favorite guitarist here) played it?
Now, you don't have to go through all the questions in one session. One day, just look at the counterpoint. Another day, the texture, etc. We want to build a relaxed awareness of the piece. I would recommend doing this work before you pick up the guitar, as the technical aspect of playing can be distracting for score study. Once you feel like you've become more aware of something, play the guitar and experience it.
There are many beautiful secrets to be found for those who take the time. The more awareness you have of the music, the more you can express it.