𝕃et us begin with a short description of that difference. A planned action is an intention, something you decided to do before speaking (Camille and I are going to the restaurant tomorrow night). A spontaneous decision refers to an action decided at the moment of speaking (I think I'll go to bed, now). You decided to go to bed just before telling me. There's a common notion that the planned future (be going to) necessarily has to be 'proche', but that's false. If it's an intention for an action in one hour or in one year, there's no grammatical difference. It is true, however, that many spontaneous deisions are for immediate or quasi immediate actions.

The pastor says that he (or if it's a woman pastor, she) plans to preach about fibbing. That pastor could very well have said, "I'm going to preach about fibbing next week," and nothing would have changed. Please remember that these are the uses of will and be going to that we are discussing here and now. There are other uses that do not concern us at this point.

Just to be sure that this is crystal clear, un peu de français: la grande distinction entre will et be going to est que le premier est utilisé lorsqu'une activité future est annoncée au moment où l'on parle, tandis que le second fait référence à une action futur qui est planifiée plus tôt, avant de l'exprimer. Une intention. Nous sommes dans la même bureau pour une réunion. Si je me levais soudainement pour partir, vous me demanderiez : "Where are you going ?". Vous ne demanderiez jamais : "Where will you go ?" C'est parce que vous savez que je me lève et me dirige vers la porte pour une raison, et vous voulez savoir quelle est cette raison. Je pourrais donc dire : "Je vais chercher du café" (une intention). Si vous me demandez si je peux vous apporter du chocolat chaud, je pourrais dire "OK, I will" (ce n'était pas une intention avant que je parle).