Please note: The Usual Disclaimer applies as always.
When creating a new character for a campaign (or for a book or anything else really), it's useful to have a fleshed-out backstory. That way, whenever you have to improv how your character responds, you'll have a thorough understanding of how their alignment and motivation will influence them, what they're going to feel about the events in front of them, and how to best respond. After all, the r in RPG stands for role not roll.
So with that in mind, here's the steps I go through and questions I ask whenever I'm creating either a player character or a larger-role NPC. Hopefully you might find it useful too.
- What's their name? Do they have any nicknames?
- How old are they chronologically, and how old do they appear?
- Physical characteristics, standard clothing and equipment
- Secondary characteristics - accent, tone of voice, mannerisms, ticks etc
- Emotional characteristics - are they prone to getting angry/laid back/investigative/despondent etc, personal prejudices?
- How do their emotional characteristics affect how they interact with people/groups?
- Alignment, motivation and character flaws/needs
- What were the defining moments/events from their history?
- What do they most fear?
- Do they have any secrets? Who have they told, if anyone?
- Where are they from? Do they have relevant cultural associations/quirks? How patriotic & nationalistic are they?
- What was their childhood like?
- Who are their parents & siblings, and husband/wife/significant other?
- Status of immediate family (including any children) and friends - alive/dead/missing
- What's their profession and hobbies?
- Do they have a religious affiliation, and if so what is it and how strong?
- What are their major personal beliefs/ideologies?
- Do they have favourite foods/drinks?
- Why are they here, specifically? What are they looking to get out of the adventure?