One of the biggest questions that plagues DM's, especially new ones is "When do I say no to a a character or concept."
This question seems hard as after all who wants to disappoint their friends or drive potential players to level griding warlocks instead of hanging out with real people. As I tell people, its not 1981 and people today have a lot more entertainment options some of which simulate the more casual aspects of gaming pretty well.
In my experience these are the best guidelines
Say no when.
#1 It was clearly not allowed up front
#2 It seems like it would take away from the campaign and impair your fun or the fun of others
#3 If you say no, say why and offer a different suggestion
As a personal example.
A GM of my acquaintance (the one whom the iconic NPC Tanzar is named for incidentally) was set to run a "Desert" themed 3x campaign with some peculiar philosophical leanings.
Interested I asked him "Sound good T. Can I play a Monk with the sand feats from Swashbuckling Adventures, a book I knew he had." It was a pretty cool concept, Sand Runner Monk and I even had aneat background for the game world
His answer after some hemming and hawing was "no."
Having heard what the others were playing and having a few other concepts shot down I thanked him cordially and I'll pass. Some time later the players informed me it was a pretty good game.
Basically what happened here is both of us lost. I missed out on a good (well minus the troubles he had with high level play) game and he on a good player. The sad part is that good communication would have made for an all around better situation.
Had I would have handled it had I been in his situation (and I have been in several like it) I would have said 'Oh Sorry T, I'd rather you play something else. The monk is a little weird for this game and the sand feats kind of defeat the battle against the sand aspect of this game. How about a Ranger or a Druid? "
My reply (and he knew me pretty well so there was not new to each others play style excuse) I'd have said "OK thats fair" found another way and we'd have both been ahead.
A note on my Spider Cult of Mirkwood Campaign posts - As you may have noticed, I’ve posted a lot about this campaign over the past day or so. The reason: my *Adventures in Middle-earth* campaign is starting up...
4 weeks ago