I chose option 6.
There's two things that might happen.
1: I don't imagine what they look like at all. This is the most common occurrence, and typically it happens with characters who just don't get sold well by their players.
2: I get some visualization stuck in my head, and there's no changing it. For example, in the last campaign I ran, there was a gargoyle druid who work bone armor and wielded a glaive. I always pictured him as his mini, though, which was a gargoyle in leather armor and wielding a bow. In the same campaign, there was a barbarian who had a full head of hair and wore various trophies (such as a mantle made of a bulette's skull). When he first came up with the character, I pictured the barbarian from Diablo II, and that's always how I saw his character despite the fact that it really looks nothing like what he described.
In either case, it's not conscious. I don't make an effort to visualize other peoples' characters... Either it happens or it doesn't.