An RPG is about people telling a communal story and having fun while doing it. If all that people do is get together to get in fights and kill things there is Warmachine or that other, more expensive, one. Players and GMs get the fun of a common goal that they work towards, the players slowly chipping away, up and down action, the GMs frantically trying to spin a story from whatever cliché they can think up in that moment. The putty to fill in the gaps between expectations and what is delivered comes from everyone buying into the premise of having fun together doing the same thing. This is the same reason why people get together to do various sports with friends, play board games, play cards, watch movies, or even just to have a few drinks together.
The difference with RPGs is that while there is a game, there are also roles to play; I mean it is right there on the book jacket. During setup everyone gets their role, depending on their desired outlooks for the game: the face, the muscle, the tinkerer, the sneaker, the mob of other people. This make up requires communication. If you’re playing a random Lord of the Rings derivative RPG and your players all have fighters, as the GM you shouldn’t spend time with a with a rogue based sneaky portion to save the party from a massive fight. Everyone ends up feeling frustrated at not being able to use their fun abilities.
To help facilitate group creation the GM can give an elevator pitch for what type of adventure is going to be happening. Even a list of a few movies or books you’re going to riff on is a good idea. A blank page can lead to anything, one person wanting to do cowboy horror and the next to create a werewolf teen basketball star, guidance is needed. Everyone sitting down at the table needs to say what their intentions are for their character.
The Long Campaign
As has been said before a miss-matched group of characters leads to inevitable headaches. I normally want to talk over with the players ahead of time what sort of campaign we all want. I may have my own ideas for the campaign, but I may be spurred on by a cool idea that a player has.
Party creation can be done in its own session and usually it helps with setting expectations for everyone. If it’s with old friends it allows for a nice relaxing time, if it’s with new people you can size everyone up and the emergence of a table leader starts to happen. This isn’t throwing away a session. You’re gathering ideas and honing them into a larger notion of where you’re going with it. It allows for the players to decide what they’re taking and to make sure that any particularly visible holes can be dealt with.
The holes matter in a long campaign, especially if you as the GM want to play around in one of the areas that the players are weak in.
Party balance is a weird beast depending on the RPG system you’re in. If you’re in a more narrative system where people are able to do pretty much anything and it’s the story that matters it isn’t as crucial. With narrative games it does still matter when considering the “odd one out.”
When I was in the playtest of @Fiddlebacks Mask of Ordo (a great module, I cannot recommend it more) we had two to three combat orientated people and a sneaker, the GM tried to let the sneaker get ahead and do stealthy things, but the rest of us didn’t let him because it wasn’t in our character’s natures.
If you’re in one of the d20 alum games it matters an amazing amount. If you are chronically without a healer that means slower going due to natural healing, no one to control the enemies for more than a round, and your damage dealers are having to evade without dealing damage. Not enough damage dealers and the fights take too long and people get bored.
One Quick Shot
One shot adventures are a completely different beast, whether it’s at a convention or wanting to try out something new. These don’t always require a balanced party, as @wood-jasond rightly points out, but it depends on what people are trying to do with it. You need to be aware of the experience you want and what you’re trying to accomplish. Is it a convention game with a bunch of hardened players for the system? Are you doing pregen characters? Can that adventure be used to welcome new people to it? Do you really want to do that one and only time you GM a zombie adventure now?
Making sure you know what you want out of the adventure allows you to give the players an idea of what is coming up and how it will affect them. Knowledge of the adventure paths allows you to also decide if player party balance matters to you or if it’s more advantageous to have a completely tilted play group. Can three fighters and a mage slay that dragon or does it need to have the rogue and cleric as well?
I’ve gone through my two types of adventure thinking here for both campaigns and one shots. I hope it helps with figuring out how much you want to know about the party before starting to plan an adventure. Planning ahead is wonderful for a campaign, but a series of independent adventures that focuses on each player can allow for greater enjoyment.