I initially resisted. In fact, I'm resisting right now, and am only writing this blog post to avoid writing another character sheet. But I'm here to tell you they are as invaluable as they are tedious.
I've been using Scrivener, which is an amazingly robust writing program that allows you to consolidate nearly ALL of your miscellaneous data (research, photos, maps and yes, character sheets) into one document. In fact, it comes with a template built in (see inset below).
While this is a great start, I'm recommending something far more involved. If printed out, the character sheet I use would take up five pages. I can't remember where I got the version I'm using, but here's a current link to it.
Detailed, huh? And yes, it's daunting, but like writing itself, once you get into it, you get more into it. Here's why they're a great training ground before you type those ominous words, "Chapter One."
1. They help you catch your breath. I've been thinking about this novel, its characters, its setting and its plot for literally years now. Have I written anything down? Not much. So these character sheets ease me into the story by way of introducing ME to all my own characters. Like stretching before a marathon, or having your salad before your meal, it's just good solid preparation.
2. They give you ideas. You may think you know what the plot is, and how you're going to get from Point A to Point B, but once these characters start taking shape, they may surprise you. You may invent details about them that lead you down whole new paths--also a good thing. Don't think of it as a diversion or something that's "keeping you from writing," because it IS writing. Plot points, motivations and epiphanies might emerge during this process, so keep an open mind and notepad.
3. They start to write themselves. As you move into pages 2, 3 and beyond on these things, you'll find yourself thinking "what is my character's favorite movie? really?" And then you'll wonder what IS their favorite movie--and why? And that leads to another character trait, maybe due to some family history, and maybe that leads to a bit about their unhappy childhood, and having one parent, and...well, you get the picture. These are your characters, so own them. They deserve to live and breathe, and the more qualities you imbue them with (and make sure it's a mix of good and bad, just to keep it real), the better they'll serve you. I think you'll find they start serving you before you're even done creating them.
I know they're not for everyone (and hell, I didn't even do them for my first two novels, but there's a lot I didn't do for my first two novels that I wish I could have done differently), especially for "pansters," but I say give one a shot. Start with a secondary character--you'll feel you're getting way too deep into their past, their social circle, their wants and dreams, etc. You'll be writing about things you don't think secondary characters even deserve. And that's a good thing--by doing so, even a secondary character becomes more three dimensional and less likely to end up a plot device or stereotype.