If I haven't made plans, then you can say either: "I will probably watch TV." OR "I'm probably going to watch TV."

Both will and going to are possible in this situation because we are predicting what will happen (since we haven't made any plans).

If someone asks: "Are you busy this evening?"

If I respond: "Yes, I'm going to the movies." I use going to because it is a plan I made earlier (before I was asked the question). - In this case we cannot use Will.