When I'm planning a novel, I have several different approaches. If I want to learn about the characters as I write, I do very little in the way of planning. I'll take some notes as I go to keep some basics straight, but I dive in and write.
For my NaNo this year, I decided I'd build my main characters in advance. I'm writing a UF thriller mystery, and I wanted to make sure I knew all of the motivations of the murderers, the main characters, and the groups working against each other during the book.
I am using a flexible template for my character concepts, and it looks a little like this:
Top line: Character's name.
Then, in this order, as fits per line: Age, Gender, Eye Color, Build, Physical Traits (Things like are they good at sports, how is their hand eye coordination, etc), Heritage (lineage, are they British, Canadian, American, are they of a notable bloodline…), Species, Talents (AKA Skills, but of high level), Special Abilities (Magic, etc), Personality type, Occupation, Relationship to Main Character.
Sometimes characters will have more information, especially if they're a primary secondary character.
On the page next to it, I write out in a summary format what their basic back story is. How did this character get to where they are today?
I will also include motivations and goals if it isn't obvious from their back story.
I try to do this with every reoccurring character. One-offs I may or may not include, depending on the importance of the character.
I try to make sure each character info sheet doesn't take me more than twenty minutes. I'll flush these out later as necessary, but these are just quick rap sheets for a character.
Most of the work of a character should take place when I write. At that point, it's just note taking.
I use these rap sheets to help me define what world building elements I will need before I start writing my book.