Fruit pie filling must be thickened. The amount of thickener depends on the type of fruit and the type of thickener. Some fruit, like apples, contain less juice, and more natural pectin. So you will use less thickener in an apple pie. Some berries, like blueberries, are so inconsistent when baked, you can still end up with a runny filling after using the recommended amount of thickener. N this happened to me on the 4th of July. My industrious friend ladled out the excess juice and used it for ice cream topping.
King Arthur Flour has an explanation and guide for the various thickeners (link below). It explains the pros and cons of the various thickeners, as well as a guide for amounts based on the different fruit and type of thickener.
i prefer quick cook tapioca or potato starch to cornstarch. I find cornstarch too gummy and cloudy.
Some pie tricks I use:
Mix fruit, sugars, spices, thickeners in a mesh strainer set over a bowl. Let sit for 15 minutes; juices will drain into bowl. Heat juice in a saucepan (most thickeners require heat to activate), then I’ll add the warm juice to the fruit when I’m ready to fill the crust.
If using a metal pie plate. Preheat a metal baking sheet for at least 10 minutes before putting pie in oven. I haven’t had a glass pie plate shattered from placing it on a hot bake sheet, but Pyrex warns against putting glass on a preheated tray.
Bake pies on lowest rack position.
Bake hot. I preheated oven to 425 for at least 20 minutes befor putting a pie in the oven. I bake at 425 for 10 minutes, then reduce to 375.