By passive and active, I mean understanding language when used vs. using it yourself, like in the terms active/passive vocabulary. My experience learning languages is that active skills are much harder to acquire than passive skills. I'm trying to explain this to someone who is learning a second language for the first time, who wants to focus on active language usage and under-emphasizing passive usage, but I think they are having difficulty because this seems backwards in terms of goals. Are there any good written explanations or academic work on the differences between active vs passive skill acquisition, or on different learning strategies for people who want to focus on one or the other?
I am not totally clear what you mean by active vs passive. Do you mean the ability to produce vs the ability to comprehend?
In general, the guiding principle in language learning is that considerable exposure and comprehension precedes productive abilities.
Of course, productive skills and receptive skills are reciprocal (just think, how can you say a word you have never heard before?) so the idea of isolating them is problematic.
If you want to go deeper into this topic and look for academic sources I recommend specifying a specific area of language (eg. grammar, vocab, phonology, discourse etc). The answer might be different depending on what you are looking at.