Lilith Saintcrow talks about this as "A State of Focused Wonder" here: http://www.lilithsaintcrow.com/journal/2010/01/a-state-of-focused-wonder/
This back-brain daydreaming is how I get to know my characters and their story. Some of my daydreams make it into scenes, most of them don't. But all of them help me to get to know my characters.
I'll sometimes free-write information about a character, sometimes it comes in a brainstorming session with fellow writers (thanks Meg!) and sometimes it happens while I'm driving to work.
Below is what I wrote after a brainstorming session for a new character in my current WiP:
Name: Walter Lang, early 50's
Career Army – Chief Warrant Officer and part of the RRT team
Archetype of the Mentor – Dumbledore or Gandalf – Walter's helped Lani, but just before they go on their first disaster, he tells her that he's retiring. She'll need to lead the team herself.
Married, his wife is Mary Alice. She's getting him to quit smoking. She's a small thing, and fierce.
Walter is uncomfortable around brass (officers), but he likes Scotty and Jerry – they're mechanics and geeks, too. But Tomas and McKittrick make him twitchy. Lani is okay because she's a civilian.
Walter is a Loggy - a logistics specialist. If you want something done, talk to Walter. The flip side is that if Walter doesn't want it done, it isn't done.
He wants to see Lani succeed. He wants to retire. He wants to pass along the torch.
And here's the bit of scene that came out of that:
The main office where we work is a hive of activity. We really don't have one office to ourselves; it's more like we've got a corner of a huge base office over by Colonel McKittrick's office. We call it the Pen. I think it's something like the bullpen. Everything around here is some kind of baseball metaphor.
There are times that it feels more like a federal pen than a bullpen; getting from one end to the other can be like navigating traffic in Indianapolis. Without traffic lights.
It was the normal chaos, with ensigns and lieutenants delivering papers and reports to the mucky-mucks. I don't think I heard anything, maybe I saw something. Over by the main entrance – the door to the outside – a tall man with white hair just stood there, looking around. At first, I didn't recognize him. I guess context does matter. It was Walter Lang and he was definitely out of his element. He had something clutched in his hand, held close to his body, and he looked about as lost as someone six foot four could look. He looked like he needed rescuing.
How do you get to know your characters?