Ok, I talked about the reasons roles exist and how to define them, now I’m going to talk about how to present them to players.
For this to work, first you need to figure out what level of freedom to allow your players. The spectrum runs from games that allow a player to build whatever character they see fit, as exemplified by games like the Elder Scrolls series and GURPS, or force them to progress along a very specific route, showcased nicely in games like pretty much every game ever without those nebulous “rpg elements” in it that still has power progression. If you’re making one of those, that’s awesome, but this isn’t the post for you.
So let’s look at an example of the middle point. Pathfinder (and several editions of D&D, the game it was based on) has a system in which players choose a class that defines the basic nature of their character. A fighter fights, a wizard casts magic, a rogue stabs people in the back and takes their shiny things. But within that structure, there exists a lot of customization options. Some classes allow you to select from a suit of abilities, and all classes receive a number of skill points to be spent on non-combat related skills, like opening locks and professional home decorating, and a number of special abilities, called feats, that they can choose from a largely universal pool. Characters are also free to combine classes, gaining additional benefits at the cost of not increasing their starting class.
The question becomes, as a designer, how much do you trust your players and yourself? A game of complete freedom might make for very interesting character creation and some very interesting characters, but you run a grave danger of players creating characters who are completely unsuited for the game ahead. For an example of this, the latest Deus Ex tried to follow it’s predecessors freeform progression system, but had segments where a character without significant combat capabilities could not progress. This could have been solved by toning down those sections, allowing a way around them using the same methods the character had been using to progress through the rest of the game, or (and this is the game design aspect we are focusing on here) they could have separated the progression of the character in such a way as to not force the players to choose between competence with weapons and the ability to read your boss’s emails.
The more freeform your creation and progression system, the more hand holding you need to do. Shadowrun does an excellent example of this by providing a series of premade characters with descriptions of how to play them and why they are effective, and the player can use these characters straight off the rack, customize them, or just use them as inspiration for their own characters. Alternatively, Warhammer Fantasy Roleplay (the second edition) forces characters to progress in very very straight lines, and only needs to provide one sample character who is only used as an example to show how to apply the character creation rules.
The opposite of this, of course, is the more constrained your character creation and progression is, the less unique and interesting the characters made in it are. Rules exist, in GURPS, the Generic Universal Roleplaying System, to allow players to create magic using dinosaurs who, over the course of the game, learn how to hack computers and craft elegant herbal teas. The level of player freedom can be as great as the player’s imagination, which simply cannot be said about Warhammer Fantasy Roleplay. But if the focus of your game is supposed to be very tight, that might be fine for you.
So remember, with great freedom comes imagination and occasionally stupidity and with great focus comes reliability and occasionally boredom.