One of my weaknesses is character dimension (according to my editor who should know!) is character dimension, particularly in early drafts and particularly with my lead characters. It gets there by the end of the revision process but it can always be improved. With each book it becomes harder in a way because you are not trying to repeat character traits that you used in other books, but to a certain extent you have to. For example, a Viking warrior is going to be far more like other Viking Warriors... But your own life experience is by nature limited and thus you tend to create characters who resonate with you. Create too many, and they will start to be the same. Or your editor will start asking why are your characters too flat, too nice or too whatever.
It is one of the reasons why I love The Manuscript Makeover by Elizabeth Lyon. In it she says that in her vast experience, the protagonist is often the LAST character to be well rounded. There are a number of reasons for this. Mostly I think it is because it is the protagonist's story and highlighting different aspects in revisions makes the protagonist be more well rounded.
Anyway, it is something I am working on.
She mentions character dictionaries -- i.e. developing a vocab for each character that reflects the character's situation. Certainly the use of a Word Menu can help shorten that process. A Word menu groups words, so you can easily find words on art history for example and if your protagonist was an art historian, you can use those words to develop speech patterns. These are different words than if someone was a photographer.
Dimension goes beyond goals, motivation and conflict. Dimension is about attitude and how the primary motivation/goal colours everything they do.
Anyway with my current revisions of the single title, I am really thinking about attitude and dimension and how to show things. How to change generic words into words that really define a character and make them memorable. It doesn't happen on the first draft. Nor second but it needs to eventually happen.