In the long term my job as a DM is to bring on the fun, but that involves (in most cases) a little collusion- talk to players, as them what they want to be doing at Paragon, attempt to find ways to keep them constantly involved with their characters decisions etc. Example- Rock (played by Akshay) Dwarf Fighter was rolled up as a Fighting/Killing machine.
At 7th Level the players were searching through an ancient Dwarven Creation Forge, and laying waste to Dwarf-forged (work it out for yourself), then they discover a whole bunch of paperwork about about a Dwarven special project, project R0, by the inventor Creator Klum. Seconds later RO-CK gets put together and the room lights up. And Akshay who knew nothing about my surprise, well he has a choice, I'm not saying is him... Me and the player have an entire avenue of 'fun' opened up. Later, after the whooping and the metal-man jokes have worn thin they discover project R1CK. And thus ROCK has more purpose than he did last week, and I have massive opportunities to foreshadow, well who knows what but I have ideas.
Farkill keeps seeing ghosts, ghosts of dead Dwarves, me and Zoran, who plays Farkill, know his backstory- only I however know how it is going to impinge on his present life. Longstory short Farkill has pissed off Moradin bad, screaming in game (RP) and cursing his God he smashes his hammer into the ground and I calmly state that 10,000 tons of rock and stone peel away and fall into the abyss- ending a traumatic/climactic encounter in a matter of seconds- sending a dozen Gnolls and more Hyenas to their instant death; and a Large Shadow Dragon in full retreat. Farkill has the power, Moradin loves a sinner- and the player, the player giggles furiously as the other players scream various epithets, all good.
Then there's...
Fun, moment by moment, everybody wants and needs that but it's hard to nail down, moment by moment. As a DM I need to know what the players like/enjoy/want; then I need to make the story their story- all of them, sure it gets twisty plot-wise but if there's something at stake then the fun/interest, or likelihood of either, factor increases.
I don't neccesarily fudge rolls for fun, or at least not often; actually almost never. I do give my players what they want- provided I can make it work, it's not too counter any of the other PCs (so our bastard Rogue still behaves terribly at times).
A moment about our Rogue, everyone loves him. In game he's a terrible man (Elf), at level 11 he's still completely out for himself, secretly (and not so secretly) reviled at times (which makes the other players laugh- he's a great 'character', the other players laugh even when they know he's robbing them). And so as a DM I ensure that everyone the PCs meet instantly trust him, because... because it just works better that way- women swoon, if the PCs save the day then the patron always thinks that the Rogue is some Paladin, or Brave-Sir-Knight. While Cathal, the actual Brave-Sir-Knight silently fumes... People trust him with their lives, live-savings and wives. And the other PCs giggle behind their hands as Lucan (the Rogue), urolls '33' Bluff and explains he is Acolyte of Ioun in search of somewhere to lay his weary head for the night, sisters (with a wink).
So fun facilitator, and the way to do it is to know what the PCs want (IMHO), moment by moment then do your job well, describe the action with a nod to the cinema and action movies. Make the plot/story immersive and involving directly every player.
Nothings guaranteed, as stated elsewhere it requires a high degree of collusion but as long as all players are agreed that they are sat around the table in the hope of having fun in the first place...
The responsibility is not solely anyone's, the DM however has all the resources to a story/adventure/scene into a memorable moment in which the fun comes to the fore.
Cheers PDR
And I haven't even touched on Farkill's Ghost-Powered Holy Chainsaw of Moradin, or how and why they found a clan of jet-black skinned Orcs (which they rescued from slavery by the Duergar); jet-black skinned save for their hands on which the pigment had worn thin- the reason, the tribe possessed (and later gave over to the PCs) a pair of Mobile Phones (no signal or battery) and yet the 'Mobies' still retain the ability to text.