WILL or GOING TO ?

[pcgasparetto]

Friends,

I know I use GOING TO when I have already planned to do something, and WILL when I decide what to do at the moment of speaking. But this is something related to "answers". What about "questions"? For example...

Which bus (1) you (2)? [catch]
The 651. If it comes late, I (3) take the next one.

1 = "will" or "are"?
2 = "catch" or "going to catch"?
3 = WILL (it was decided at the moment of speaking, right?)

What (4) you (5) tonight? [do]
Maybe, I (6) go to the theater.

4 = "will" or "are"?
5 = "do" or "going to do"?
6 = WILL (the word MAYBE shows that it was decided at the moment of speaking)

So, please, help me understand this... When it comes to "questions", how do I use WILL or GOING TO correctly?

Thanks for your attention. :)

PC

[Casiopea]

Sam: Which bus are you going to catch? <have you even decided yet?>
Max: The 651. If it comes late, I will take the next one. <definite decision>

Sam: Which bus will you catch? <You've decided, right?>
Max: The 651. If it comes late, I am going to take the next one. <It's a plan>

Does that help so far?