There was a thread on a writing forum I frequent that asked others if they can picture their characters, and the OP stated that they have a hard time doing so. Many replied that they do not see their characters, or can only envision a few aspects of their characters, such as the color of their hair, or physique of the character's body, or their sex, and what not. I answered that I could indeed picture my characters and that I have drawn them from memory many times, although the drawings don't always come out like I want them to.
The majority of the respondents said that it didn't matter if they could see their characters, what was important was what the characters said or did in the story. The reader in their mind's eye will picture them as they saw them anyway, so a few modest details was all that was needed. I believe this to be true. I do it all the time. I like to be given a few details of a character and be allowed to see them how I see them. Some writers even said they don't describe their characters at all, unless the detail is important to the story. But when it comes to historical or fantasy characters, I like to be given more detailed descriptions, including their clothing and hair or scars and what not. Or if the character is non-human, it's nice to be given a detailed description of what these beings look like, but of course, I don't want the info dumped on me all at once.
However, there were those that said they can't envision their characters or those in books they read, even when details are given. For me, I find that strange because I can always see a character in my head as I'm reading. My reasoning is that I am first and foremost a visual artist. My interest in drawing had come way before my interest in writing, so maybe that's where my ability to see characters comes from. Give me a few choice details and I can draw you (or, at least back when I did draw) a picture of how I see a character. I've even posted a couple of drawings on this blog of my characters. Drawing characters was more effortless for me than to use words to describe them. Although, nowadays the opposite seems to be true.
So how about you? Can you see your characters clearly, or not at all, or somewhere in between?