Main principle: Future Continuous Tense shows that an action will be happening over a period of time in the future.
We’ll be watching movies tomorrow.
In non-continuous tenses like Future Simple, the action is not ongoing.
They will accept our offer.
They will go to the store tomorrow.
Fact of action vs duration of action
We will watch movies tomorrow. (fact of action)
We will be watching movies tomorrow. (duration of action)
But how do we know that there is a length of an action, that an action is continuous, ongoing? Well, for this we normally have explicit or implicit indication of duration period:
Tomorrow (meaning a significant or some part of it), all day, all morning, for a few hours, from 5 to 6, during his visit, etc. etc.
Anything that can show duration. If we don’t have an indication of duration, we infer this duration from the context. We call it a contextual point or period in time.
Also on Future Continuous: