My name is Manuel Diewald and I am a Sydney-based guitar teacher in West Pennant Hills with a B.Mus. (Hons) (First) in Music Pedagogy from The University of New South Wales (UNSW).
When it comes to dynamics and phrasing in musical passages, we tend to learn at an earlier stage of our musical development that one or two dynamics are generally used in a phrase and that we maintain that dynamic until a new dynamic is indicated. As we continue to develop, we need to start looking at a phrase with an intended general dynamic but at the same time allowing for variations within that phrase to give more shape to the music.
For example, a phrase can be indicated as mezzo-piano though we can shape this phrase even more by thinking about the intended mood of this section and the direction of the pitch of notes. If we take this excerpt A below from Sylvius Leopold Weiss' Ciacona, we can see that there is a lot going on in terms of dynamics and phrasing.
Excerpt A (Ciacona by Sylvius Leopold Weiss)
All these markings have been added in by myself and are my own musical interpretation. I am not saying this is the best or only way to play it but this is how I interpret this piece. I have indicated the phrasing with the markings above the music and have added in the dynamics below along with some other musical expressions. This whole section is in mezzo-piano though within each phrase there is a bit of a crescendo leading up to the highest pitch in each bar following the music's direction, in this case, the pitch of the notes are ascending. I also speed up slightly and then slow down within each bar of the 2 bar phrases to further shape the whole phrase. At the end of each 2 bar phrase, I do give a bit more of a rit and come down in dynamics as well to emphasise the end of the whole phrase. As you can see, all this information written on the music can be a little distracting, at least for me and so this can be a reason why composers did not write this in. Also in the Baroque era in which this piece was composed, it was expected of the musicians to know how to play it and so composers did not feel the need to write them in. You may want to add your dynamics, however detailed they are, initially so you have a good understanding of what you want to do. When you are comfortable with your interpretation and know it by memory then you can always remove them (always mark your music in pencil). When it comes to the tempo of the music, we are using a term called rubato. Rubato, which means 'stolen time', allows us the shape the music by pulling and pushing the tempo (not referring to the dynamics) allowing us to better shape the music along with the dynamics intended.
Sometimes the details of how we phrase our music can be quite complex and intricate though do not be discouraged by this as the end result leaves us with music that has a lot of emotion, character, and overall beauty that allows the performer to express the composers and their own expression. This detail is partly due to what makes a performance of a piece so stunning and spectacular. The other would be the performer's technical and expressive abilities to portray them.
I think the best approach to achieving this kind of detail is by starting off simple and adding layers as you go. Begin with a general shape to the music by identifying the phrasings and what direction the music is taking. Start with a general dynamic and then see where you may want to be louder or softer and whether you want to speed up or slow down. This way it is easier to grasp and take in the finer details when you add them. Remember, not all sections of a piece of music need to have this much detail, and some pieces lend themselves to fewer details. It all comes down to the composer's intentions and putting your own spin on them while respecting the composer's decisions, at least that is my opinion on them.
I encourage you the next time you practice to take a piece of music and apply this approach to how you bring out the musicality in the piece you are playing.