I keep a writing diary and note down what I have written, sometimes what I am planning to write, and use it to note deadlines for competitions too. I've found it helps me be more productive. Some writers also keep a character diary. I don't do this myself (as yet, anyway!), but can see the point of it. You can reveal more of that character in a diary and include things you couldn't include in the main story (because it would have distracted from the real plot and so on).
A character diary, even if you never share it, could be a useful way for you to find out whether you know as much the character concerned as you thought. Don't be surprised if you end up being surprised! I kind of did this in drafting my novels (and learning a lot about the craft as well as about the characters in the process).