I know several questions were asked about the difference between "going to" and "will".
Based on several answers (see, for instance, here, here and here), I understood that "will" is more spontaneous and "going to" is used with more planned actions.
So, it seems that everything is pretty fine. However, in this question, Kosmonaut has an answer in which he states:
"Let's say that tomorrow you will walk your dog from 7 – 8 AM".
On the one hand, you probably planned to walk your dog long before and thus I should use "Let's say that tomorrow you're going to walk your dog from 7 – 8 am".
On the other hand, since I'm saying "let's say…", I'm deciding right now (thus, unplanned) that you will walk your dog. So, even though in this hypothetical situation you made a plan, I'm in a more spontaneous mood deciding right now that that's what you will do tomorrow, and, thus, I should use "will".
Which one (if any) of the above explanations is right?
Best Answer
In most contexts, I am going to [verb] and I will [verb] are interchangeable. Sometimes the former may place more emphasis on the fact of your current intention/expectation, where the latter emphasises the future action.
There are some contexts where the difference is clear, and this may have some bearing on why OP thinks there's a planned/spontaneous distinction (usually there isn't). Say you're round a friend's house watching a football match on TV, and at half-time the beer runs out...
If you reply "Don't bother - I'm going to use my car", the implication is you had already decided you were going to use the car before you first said you'd get the beer. But if you say "Don't bother - I will use my car", this implies you just made that decision in response to what your friend said.
@Peter Shor mentions another context ("It's going to/It will bite!") where native speakers often make a distinction. In that case, going to usually signifies immediate danger (it's just about to bite), but will can just mean it's bitten others before, and will/may bite you soon if you're not careful (effectively, the same as "Careful! It bites!" where present tense indicates "habitual" action).
Kosmonaut's "Let's say that tomorrow you will walk your dog" isn't really relevant to the current issue. It could just as well have been "...tomorrow you are going to walk..." or even just "...tomorrow you walk...". They all mean the same, and in that context there's no real reason to prefer one over another.