Generally speaking, at least in my experience, there is one of two "rules" that someone asking for 'basic advice for the new GM' will be told. The first is that they have to be able to say no to their players if they don't want their game spiraling out of control. The second is that they should never ever say no. At first glance the two seem to be exact opposites, and I've seen it confuse more than a few first time GMs, so today I figured it might be fun to go over it and break down how the two rules not only don't conflict, but actually can work very well together.
First off, let's get the full text of the rule as I've heard it. A good GM must be able to say 'no' to their players. Now, first off, the key word in this sentence is right there at the end. A GM has to be able to say no to their players. The importance of this will come up later, but just remember that I made a distinction for players.
Is this BS? No, it isn't. A GM does need to be able to say no to their players. As the GM you are effectively in charge of the game. Yes the game is everyone's and the story in the game is everyone's but you are the authority figure, the custodian, and - yes - the protector of the game. It is the GMs job to make sure that the characters in the game can not only work together but also work with the plot and themes going into a game.
To give an example, one of the most successful games I've ever run was a Super Hero game. One of the biggest failures I've ever run was a Super Hero game. The sad thing is the failure was much more recent. I know how to run super hero games. I'd even dare say I'm good at it. However, with all the successful games I've run I've told my players no. I've told people concepts weren't "heroic" enough for what I was going for in the game and what people said they wanted. I talked about more gray area concepts with the player to make sure we both knew where the line was, what I was expecting, and would effectively be a "deal breaker" for the character remaining in the game as a PC. In the big failure, I didn't. I let whatever the concept was through and in the end we had a bunch of players playing characters that no matter how I manipulated events just couldn't and wouldn't stay together as a group or come out and act as super heroes. It ended up killing the game.
So, yes, a GM needs to be able to say no. Being a GM isn't easy, and despite what some people will tell you it isn't the actual running of the game that is hard (though that can be tiring.) No, the hard part comes in with telling someone they can't have something, they can't do something, or that they are no longer welcome at the table. It sucks, but it is part of what comes with the GM screen and all the NPCs.
Never Say No
The second piece of advice goes like this. Never say no in game. Say "yes, but," "yes, and," or "yes, if" but never say no. Like above, the key part here comes in the target of the advice. For this it is in game as opposed to the players themselves. In other words, a GM should never say no to a character, but may have to say no to a player.
The idea behind this rule is to not cut down options. If a player wants to jump onto the back of the manticore and you don't have a rule for it you shouldn't tell them no. No is a hard answer. It is a wall that cuts off options, breaks people out of the moment, and can kill fun. Instead you should think of ways to make it work. Maybe you'll allow it if the person gets a really high acrobatics and a really high ride roll. Maybe you'll allow it, but it will prevent him from doing anything else while he is on the back of a rampaging manticore. Maybe he can do it and she is now exposed to archer fire from above. Not a no, just a roll with it and keep going.
More Like Guidelines Than Rules
The second thing you need to know about these 'rules' (after the rules themselves) is that sometimes they don't apply, or they apply in weird ways. This is more true for the second than the first, but sometimes things happen. For example, if we were playing in a modern day "real world' type game and a player asked you if they could fly you would likely say "no." Why? Because their character is incapable of flying. However, you could also say "yes, if you get on a plane you can fly."
The point here is don't let following these bits of advice get you into a bad situation. Some situations are just absurd, extreme, or otherwise just one of the numerous places where a rule doesn't quite work the way it was meant. Just keep the concept of both rules in mind and you should do fine.
What were the concepts?
Rule 1 Concept: You are the custodian and protector of the game, so sometimes you have to make the hard decision and that includes not allowing characters, concepts, or builds that can break the game or that will cause problems down the road.
As a benign example, in Mutants and Masterminds I tend to disallow powers like Mimic and Nullify (at least as they are written) because they involve playing with how many points a character has "active" at a given time and create a lot of work. That work tends to break combat by ruining the flow and jolting everyone out of the moment, so I just don't allow them for PCs or when I do them I do them using different, simpler, and quicker rules.
Rule 2 Concept: Don't shut down creativity or the characters in your game. Find a way to roll with their idea or what would be needed to do it. If nothing else, you can just assign a very high difficulty and call it good.
What are your thoughts on this matter? I'm curious to hear them. Sound off in the comments.
Oh, and as an aside, for at least this week, likely all of December, and perhaps for now on I'm going to try to stick to a Mon/Wed/Fri schedule. It seems to work a bit better for me right now.