I'll jump in here too. Ditto to everything PC said, especially about characters. He and I once wrote a con game with 6 clerics of widely varying faiths and personalities. The game played itself! Plot, schmot.
I'll put in a plug for having a ticking clock. You don't need to shove players along, but you do need to know what is happening in the world and in the bad guy's plans. If they want to debate plans for hours, they can, but in the meantime the world is moving along. Surf the web for clues, talk to the sage all day, but the embassy is still getting attacked at 2 am, the ritual is still taking place at midnight. Know this, and you have the lever that moves everything else.
I'll also plug the "guy with a gun" theory. Always have a way to get the players back to the plot. The best way is to bring the danger to them. In a tournament game, the water should always be rising. They can debate strategy all they want, but if they don't start bailing, they'd better know how to swim.
I have a preference in Call of Cthulhu, at least, for hitting the party hard at first with something off-putting. It doesn't have to be big, but it has to shock them. A pool of blood where no blood should be. A rough and unpleasant encounter with an NPC, just something to shake them up enough to know that bad &#$@ is indeed going on. I like keeping my horror players just that little bit off balance. If they can't find their footing, the can't get bogged down.
Finally, remember the 4 rules of Con games:
1) You don't have time for this (whatever it is)
2) Keep it fast and keep it moving
3) Reward creativity and role playing
4) Less is more