Character dimensions -- trying to explain

As a few people didn't understand what I was talking about yesterdya.

One of the problems with protagonists is that you can be so focused towards moving towards the goal/exploring the conflict etc that the protagonist seems flat, like an actor rather than a person. The personality is missing.
Equally you might know lots about a character but unless it is down on the page, your reader won't know. Been there, done that. In my head, but not on the page = flat character for reader.
More than stories...