Most of the time you can allow your reader/movie-goer to fill in the gaps and your movie will be everything you both are quite happy with, but where the reader's imagination might hit rather off in left field is when the lack of description leaves them to assume. I'm reading a book (yeah, most of these posts come because of a book I'm reading) where the 'people' aren't the normal kind of human you and I see every day. With Trolls as a part of this world, I can accept that things are going to be very different. The fact that much of society is dictated by a 'crest' grown at puberty, also tells me that the 'people' aren't quite normal.
The thing is, with this book, I don't really know what this crest - any version - looks like. I have no problem picturing the Trolls - they're various degrees of big. They have tusks. They have three fingers and a thumb. They have huge feet. They stink. But appearances can be deceiving - they are not stupid. One race - crystal clear.
The crested race was also perfectly clear - all except the crest. Bald until puberty. Five fingers and a thumb (assumed). Standard facial features (assumed). Two legs and two arms (assumed). A full range of body sizes and shapes (assumed). What lead all of my assumptions? All the pronouns - he - she - woman - man - you get the idea; we use them all, all of the time. They lead me to picture yo average Jo or Josephine of whatever position in life. The fact that growing hair was the height of bad luck, and that such a person would be utterly shunned was brought up early on, as was the waiting for the cresting. The main character is 14 and his cresting is late. He's VERY anxious about it. To remain a 'moon-head' for the rest of his life was only a little better than growing hair. What teenager doesn't worry about how they are perceived by those around them?
So what does this crest look like? It will decide his standing in his society. It will influence the standing of his family. Above all, it will dictate what class of magic, if any, he'll be able to access. At first I thought it might be something like a stiff mohawk only maybe stiffer. They come in all shapes and sizes, and there was some mention of temper engorging them will blood so I picture something like a rooster's crest. However, a comment of 'running fingers through it lead me to think of something more spiny. I mean, all the other details have to still apply.
Having found this book on Twitter, I asked the author about this. His vision was something primal to the Minbari from Babylon 5. Interesting. Now I have a new detail to add to my still-blurry view of these crests. Ah but this is really only a minor detail. The kid's crest is supposed to be something unique; I'm sure I'll get a proper focus eventually. So, stay in focus. Questions, distract your reader - remember that.