When playing trills make sure that the lower finger contacts the string firnly enough and the trilling finger lifts to a consistent height.
Lift the finger up high at quarter tempo. Although the finger does not lift as high at tempo it must retain the energy of lift needed for clarity.
Look for the optimal hand balance to support the trilling fingers. In this clip since the 3rd and 4th fingers are not used, the hand is centred around the 1st and 2nd fingers.
Practising in three tempos one after the other is useful so that we can immediately check that the action we have programmed at quarter tempo works at tempo.
Slow practice is good for diagnosis and programming. Fast practice is good for checking that the new skill works for patterning (thinking in large units) and for preparing for performance.
Finger Action - Introduction
Finger Action - Lizard Tongue