I think that ease of reading body language may be exaggerated to some degree in fiction, as minim-calibre says above, just ot make things easier on both the reader and the writer. However, I also think that there's some literary convention going on here. A phrase like "His eyes/face darkened" is not, I think, intended to convey a literal darkening but a metaphorical one--and it's a lot more concise than saying "His brows came a quarter-inch closer together, his forehead wrinkled, his lips thinned, and the corners of his mouth kind of pinched up a little, causing his labionasal folds to deepen."

I think that it's easier for people who know each other well to interpret body language, because there's a huge amount of context for it--you know how the person has reacted in many similar cases in the past. I know that my girlfriend and I can 'talk' in body language--not hugely complicated stuff, but stuff "I like/don't like that," "I'm happy/sad/bored/etc," "What a hoser!" "Good God, will she never shut up?!" I doubt I could do the same with a stranger; I wouldn't know what the cues meant for that person.