Rhythmic control of bow and left hand
Rhythmic control is what enables us to play well with others, to shape phrases, and even to delineate characters. A violinist without a good sense of rhythm is devastating in any type of ensemble. An understanding of pulse and how to develop your own inner pulse is crucial, and there are many useful exercises in rhythmic accuracy for both the bow and the left hand. Evenness in technique and rhythm is essential.
However, once we have mastered this basic requirement we will also need to learn the appropriate use of rubato, both subtle and large. A small placing of a note can emphasize it, just as does the placing of a word in a sentence, and a slight sense of forward motion in the middle of a phrase can give it breath and life. This sense of organic timing is what ultimately will give our playing artistry and magic.
Note well that it is far too easy to mistake a lack of rhythmic control for musical creativity! Picasso had a complete understanding of and ability to draw in the classical style before he began his cubist works. So too must we be able to play with utter stability, so that when we choose rhythmic freedom it will be for a musical reason.