Martelé and pizzicato are effective ways of training clear and in-time finger actions. The finger must be down on the string before the note is bowed or plucked or the note won't have a clear sound.
Drop each finger to the string with a clear tap. Do this rhythmically, with the tap coming halfway through the beat, like an off-beat note. Also make sure the shifts get to the new position well in advance of sounding the note.
Scores - 3-8ve Arpeggios - violin & viola
Finger Before Bow
Think Ahead for Left Hand Clarity