Seriously, I don't know why every writer doesn't do this. In the most general sense, you're setting baselines for what your characters are capable of. What kinds of skills can they generally pull off? What will they never fail? What are they particularly (and often hilariously) poor at?
Give me any genre or class of book, and I can build you a set of characters and a plot using a role playing game system.
Think about it - you're writing, let's just say hypothetically, a mass market romance novel. Obviously, we're going to use Spycraft 2.0 rules since they have some some seriously entertaining rules for Seduction.
Here we go ...
So, you have Michelle, the protagonist. She's trying to work her way up to being junior partner at the law firm Wirth, Roth, and Canvendish. So, let's make her a ... Level 2 Advocate. This gives her some nice core abilities that focus on character traits like persistence. When the going gets tough and she's about to lose the man of her dreams, we don't want her to give up. This class also gives her bonus Contacts, which are great for pulling in minor characters she knows who can help her dig up the dirt on Mr. Right to give her an excuse to push him away after letting him into her heart.
She doesn't consider herself to be uncommonly pretty, but she does have a pretty keen intellect and a certain amount of natural charm. Then again, this is a romance novel, so we're setting the baseline pretty low on intelligence, as characters go. We've got points to spend, so we'll give her a 12 intelligence, 14 charisma, and spread the rest of the points evenly between the other stats.
For skills, she's definitely going to need Bureaucracy if she's going to try to navigate the dog-eat-dog world of corporate law. We'll spread some points into the other class skills, but we also want to make sure to put a couple points cross-class into Acrobatics for, you know, the sexy factor. Also, she needs that thing that makes her special, so we'll give her some Intimidate, because burly Mr. Rights have to be strangely turned on by women with some fire and brimstone in their hearts.
Now, she only gets one feat, so we'll have to pick wisely here. The combat feats are tempting, because there's nothing I'd like more than to give her Dirty-Fighting Basics so I can throw a scene in where she punches Mr. Right in the nuts. Resisting that temptation is difficult, but there are plenty of other ... eh, decent option. We'll give her "Silver Spoon" which, for the purposes of the novel, we can define as her having inherited a shitload of British Pounds (yeah, check it, we're in England now!) from her great-aunt, Lady Anne Warshire. So, here's the catch - that means she doesn't even HAVE to work. She's choosing to because she has to prove to herself that she's somebody. Then, of course, once she's done that she can find Mr. Right and settle down to fawn over him 24/7.
Am I being too cynical about romance novels?
Mr. Right, aka Trent Reginald Harthwait III
If you're looking for a rough-and-ready, jaded, surprisingly knowledgeable, hardworking superhunk, look no further than the Explorer class. For fairness, we'll make him level 2 as well. We'll spike his strength, constitution, and charisma, and let the other stats be average.
If you were going to make Indiana Jones in Spycraft, your book would magically open to the Explorer class and all the other class pages would glue themselves together so you couldn't look at them. They're rugged and well-traveled, but they never shirk on hitting the books when it's time to do some research. And maybe, just maybe, when they have a little downtime they aren't afraid to explore their softer side and pick up a novel or watch a romantic comedy with a certain lady-friend.
Obviously he's going to have massive skill points in Athletics. We should give him a bunch of Drive as well, since it'll be important for him to full-throttle through the streets of London when he gets the call that Michelle has been in a car accident right after they have a terrible fight over some as-yet-to-be-determined misunderstanding, probably involving his sister visiting from America. He needs to Notice, so he can pick up on the little details of her appearance, the finer things. Now, knowing that he's going to get into a seduction conflict with Michelle, we'll have to break character a little here to make sure we take some Impress as a cross-class skill, just so he's prepared for the future.
For feats, "Submission Basics" is tempting, but then we're moving into a whole different breed of, uh, "romance," so I think we'll steer clear of that for now. "Whip Basics"? Damn, same thing. "Contempt": nope, still on the S&M there, sorry. "Hammer Basics"? Annnd now we're skirting into the X rating. "Style Over Caliber" - it's not the size that counts, right? Sorry, really, I can do this all day. Okay, seriously, he needs something that's going to give him an advantage in a Seduction conflict, because otherwise he's never going to be able to land that intriguing creature named Michelle. So we really have only one choice: "Total Control" - ha! Sorry, had to throw one more S&M joke in there. Okay, seriously now - "Silver Tongue" it is. Why are you still laughing, it's ... oh, I see it now. Nice.
Well, as you can tell, at this point I've got 75% of the ... for lack of a better term, we'll call it a "plot" even though, again, this is a romance novel. Most of the rest is just sex and arguing, but if we really wanted to we can map out the progress of the Seduction using Spycraft's Dramatic Conflict rules. In Dramatic Conflicts, the characters choose Strategies which both have mechanical and roleplaying benefits (or penalties).
So, in the first round, Trent and Michelle meet at a bar. They're with friends, and of course have no idea how important they will one day be to each other. All they know is that there's some sort of chemistry there. Trent is the aggressor in this relationship, so we'll make him the Predator and Michelle the Prey.
Michelle: "Call Me" - she isn't so sure about him yet, and wants to drag things out without pushing him away.
Trent: "Hard Sell" - he's leaving the country tomorrow and doesn't have time to mess around.
Michelle wins the roll, and puts Trent off until tomorrow. He's intrigued enough that he decides to delay leaving the country for at least a few days.
Trent, as ordered by the dice, calls Michelle the next day. She's already starting to regret last night's inebriated dancing, so she's going to be a little more hesitant.
Michelle: "It's Not You, It's Me" - she's got her career to focus on, she just can't right now
Trent: "Wine and Dine" - how about lunch at Le Chateau Brillion? No commitment, just food.
Trent wins the roll with a couple raises, takes her to a very nice lunch that goes fabulously well, and improves her disposition towards him while also pulling her emotionally closer. As an added bonus, his Silver Tongue deals Wisdom damage to Michelle, leaving her head feeling a little muddy every time she's with him. Could this be love?
I'm not going to keep going through this, because I think you get the picture, but I just wanted to throw one more out.
Trent again tries to push things to, hmm, completion, by going straight for "Seal the Deal", but in a stunning twist Michelle wins with the "In Too Deep" Strategy (I'm not making this up, I swear) and their sexual encounter leaves Trent wanting more from her. He becomes the Prey, she the Predator, and they conclude the Seduction on her terms. Everyone, reader included, goes home happy.
I know I make a lot of fun of romance novels in here, hopefully fans and writers of said novels will understand it's all for humor value. I've spent more hours of my life than I care to admit reading Harlequins ...
Oh, and thanks to http://game-icons.net/ for the awesome pictures, made available through a Creative Commons license.