As we’re writing a first-person/subjective narrator we can hint at the narrator’s past relationship or great knowledge with asides rather than outright telling the reader what they know.
A basic aside might be ‘I’d heard that before’, do they mean in a general sense or from that character in particular? We don’t have to tell the reader explicitly we can let them decide for themselves and either reveal the answer later or leave them wondering, though by the end of the story they will likely have made up their mind even if we haven’t revealed the answer. For instance, we may not have said that the protagonist and antagonist have known each other for years but we may have built up an interaction and hinted in the narration that they have.
An alternative may be that rather than suggesting a history with another character they can suggest they know what they’re planning or what their angle is. This could be the general implication of ‘I’d heard that before’ perhaps suggesting they know the other character is lying, or at least not being entirely truthful. The other character might’ve said something like, ‘nothing to worry about’, to which the narrator thinks, ‘I’d heard that before’. We may immediately infer that they’ve heard it before when there was something to worry about but that the other character doesn’t want to worry the narrator. Thus also implying something about the other character, perhaps that they’re trying to be nice or they’re a little prone to panic and when they feel panicky they say ‘nothing to worry about’ or numerous other things.
These layers might sound tricky, especially if we’re trying to find the right balance between enough asides and too many asides, but most of the layers won’t be there in the first draft. They build up during editing when we add interactions to show more or take away interactions that reveal more than we’re ready to at that point. The important thing to remember is that as we’re writing the first draft we’re working out the story for ourselves so it doesn’t matter if there seems to be a lack of depth, the first draft is the foundations upon which everything else is built.
Of course, once we’re considering a narrator’s asides to the reader then we also have to consider how much information is too much. Although there is a lot that can be inferred there doesn’t need to be some solid information too which can actually be more difficult to do than implying things. Why is this? Well, people will infer things whether we mean them to or not but they can get very frustrated if we give them too much information and not enough room to infer. So how much backstory is too much backstory?