I'm presuming that by this you mean will + simple infinitive and will + progressive infinitive.
You need to look first of all at the meaning of will
- which may be prediction or volition but is not necessarily future : this post
With these infinitives the action may be either present or future - but that's determined by the context and, if signalled linguistically at all, it's done by a time adverbial (at the moment / tomorrow
etc). "Will" itself is always present - it has the meaning I predict
or I intend to
- in other words it expresses present prediction/volition regarding another event which may be present, future or (with the perfect infinitive) past.
Then think about the simple and progressive infinitives : the simple infinitive expresses an action as "complete" while the progressive infinitive sees it as ongoing.
So : you have four possible combinations Prediction + on-going event : John's on holiday at the moment, lucky thing. He'll be lying on the beach at the moment while we're stuck here at work. (present = I predict he is lying...)
At this time tomorrow, we'll be flying home. Prediction + "complete" event :
(Knock on the door) Oh, that will be
John. (present = I predict that is John)
I expect we'll arrive
about 7. Volition + on-going event : Right, I'll be going then.
(= I intend to go now)
This one is, I think relatively rare in comparison with the others, and is always an alternative to the simple (Right then, I'll go now
) but that's just a hunch which may be wrong. Can anyone think of an example where the progressive would be essential? I also can't think of any examples where it would be used to express volition regarding a future event. I usually present it as an option, used when the speaker really wants to stress the "on-goingness" of the action. Volition + "complete" event : Is everyone here? OK, I'll start now. (= I intend to start now)
I'll call you when David arrives.