When pitting fresh cherries, you will inevitably release a lot of liquid. The tapioca flour will help the liquid thicken, however if you prefer a thicker pie filling, you may want to drain off the liquid before adding the tapioca flour. Alternatively, you could drain off all of the juice and cook it down in a saucepan over low heat with about half of the sugar to create a thick syrup. Then, once that syrup cools, mix it back in with the cherries and remaining sugar when you add the tapioca flour.