• I will watch a movie.
  • I am going to watch a movie.
  • He will play soccer.
  • He is going to play soccer.

What's the difference between WILL and BE GOING TO?

Short Answer: Not much. They can almost always be swapped and your sentence will still sound just fine to a native English speaking American. (I'm not sure about British speakers.)

Exceptions:

Request vs Question

  • Will you open the window?
    • I am asking you to open the window.
  • Are you going to open the window?
    • This is a question about your intentions. It is not a request.

Offer vs Statement of Fact

  • You seem a little drunk. I'll drive you home.
    • This is a polite offer.
  • You seem a little drunk. I'm going to drive you home.
    • This is more forceful. You won't let them say no.

Long Answer: The internet and grammar books are filled with long answers. Here is one and here is another. Although, with the exception of requests and offers, I would not be surprised to hear a native speaker exchange BE GOING TO for WILL in all of the examples. I don't recommend spending too much time studying these grammar rules, since using the "wrong" form will not affect your ability to communicate in English.

Soft Rules

Here are some more examples of when we usually use one instead of the other.

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New Plans vs Prior Plans
John: I am going to the mall. (Prior Plan)
Jack: Really, I will go with you. (New Plan)

John made his decision earlier in the day so he uses BE GOING TO. Jack made his decision during the conversation so he uses WILL.

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Sure vs Unsure Statements
I am going to have a baby. (Sure)
I think the baby will be a girl. (Unsure / Prediction)

Note: be careful not to mix them: I will going to.