I have a group of students who take the task of learning to play the violin seriously. We spend lessons disciplining left hands and bow arms, and I try to instill levity into the process. I try to make the work fun, because I actually enjoy work.
(Sometimes I liken training the fingers of the left hand, particularly the pinkie, to training a dog.)
(It usually makes my students smile.)
Once a student gets the upper hand on the technical stuff in a piece, and once a student is able to make a beautiful sound (I believe that a beautiful sound can happen from the beginning) at will, it's time to exercise the musical imagination.
I find that simply asking someone to play with imagination doesn't generate much in the way of musical excitement. Perhaps one of the reasons that children like to play with toys is that toys act as a springboard for the imagination. A toy can act as an object outside of the self that can be used to create a unique narrative. I suppose that even an imaginary friend or a character in a story can fill the purpose. The instrument and bow are tools that can act as toys as long as they are put into a context, but learning how to instill a sense of play in music making is what really makes for individualized performances and honest personal expression.
I suggested to one student that approaching a performance is kind of like starting a game in a team sport. We used basketball as an example because she likes to play basketball. At the beginning of a game everyone knows the rules, everyone knows the part of the court they are responsible for, and everyone has the necessary equipment. Once the game begins it becomes exciting because of the play that happens spontaneously when the elements (and people) interact. You cannot force excitement in a basketball game (or any other game), but you can allow, through the interplay of elements, excitement to happen.
When you are having fun, people who are watching the game are having fun.
It's the same with music. The elements in the music: the themes, the phrases, the articulations, the dynamics, the harmonies, the dialogues, and all sorts of other unnamable elements enter onto the "court," and the result is play. We toss melodies back and forth, we reach for high notes, and we execute difficult passages with precision. We follow the natural rebound of the bow on the string (or the tongue on the airstream), and we extend our awareness way beyond what is right in front of our eyes. The more awareness we have of what is happening in the music, the more interesting and exciting it becomes to play it.