Many "how to" resources include character sheets to help the author develop primary and secondary characters for fictional works. The author fills in the obvious physical parameters (height, hair and eye color, etc.), education, interests & hobbies, favorite color, and so forth. Then there are the deeper personality aspects. What skills to the characters bring to the story? What are the main characters' weaknesses? What are the characters' greatest fears.
I like to locate pictures to represent my main characters. Sometimes the picture inspires the character. Sometimes I have a vague idea for the character and locating a picture helps me complete the personality.
I came upon these pictures decades ago:
This is Aisley, the protagonist of Legacy, my current YA project. These pictures created in my mind the image of a slender, petite young woman who inspired the loyalty of battle-hardened men. On the order of Joan of Arc. But my rendering of the circumstances in the world of my making.
As to creating a world for a fantasy or science fiction novel, I highly recommend visiting Patricia C. Wrede's "Fantasy Worldbuilding Questions" at the Science Fiction & Fantasy Writers of America web site.
Another great site to assist with world building is Elfwood. Check out the Fantasy Art Resource Project (FARP), in particular, the contributions from Emma Lydia Bates and Michael James Liljenberg.
When creating a world and culture...I prefer to cheat. That is, I research historical time periods and locations to help answer the questions posed by the above-referenced world building sites. Now, I always thought that this was a pretty obvious approach -- until a fellow conference attendee asked how to go about creating a setting for her fantasy novel.
I like to do my research before I begin writing because I often stumble across social, cultural, historical tidbits that spark ideas for the story. A situation or event I wouldn't otherwise have considered.
And sometimes I research a location or time period that seems totally unrelated to the intended setting for my story. Most recently I read about 1950s Havana for a "space opera" story that I've been mulling around for a long time.
I'm currently at a standstill on Legacy. I suspect I'm over thinking the scenes and I should just put words on the computer screen. Right, Em?
But what I find myself doing is moving on to another story idea. Outlining scenes for Water Tribute, or doing research for another story idea like Resistance Fighters. Then when I return to Legacy, it's fresh and interesting again.
Really...writing a novel is not for the faint of heart.