I apply the three stage methods to write down the plot:
1st stage: rough plot, rough character design, story in two pages, with main points and main interactions between characters; (crappy)
2nd stage: refined plot, refined character, story written in five-six pages, with more details and some modifications; (almost good)
3rd stage: writing the story down, modifying it while writing; (final revision)
I can produce a good example with my current project plot:
Tryadine Effect, Day 31st stage:
Veckert and Blame (the two main characters) are sent to investigate District 3 warehouse in search of smugglers. They find a hidden warehouse and meet the smugglers. Suddenly Noctiphages (i.e. monsters) attack and kill some of the smugglers. The so-told smugglers' leader tell you something about Forgotten Mansion, then a Noctiphage crashes a window, enter the room and kills the leader. You have to escape in the given time limit.2nd stage:
Veckert and Blame aren't alone. They take twenty policemen with them (more realistic). They search the warehouse, when suddenly lights go down and creatures attack and kill most of the agents. Veckert finds smugglers running away and join with them in order to survive. They reach a safety room where waiting for the dawn, but when the leader tell you something about Forgotten Mansion, a Noctiphage break into from ventilation duct and kills him3rd stage:
Same as second stage, with some more modifications. Veckert finds the smugglers before
the Noctiphages' attack, and don't head for the safety room. The smugglers' leader is now killed by a sharpshooter which shots from a nearby roof while Veckert is receiving information about Forgotten Mansion.3rd stage
is actually the conclusion of a looong trail... Usually, 1st and 2nd stage take about a couple hours, while 3rd stage can require days and/or weeks...
As for telling the plot to the player... well, they usually have to figure it out themselves by talking with NPCs and gathering information here and there... usually, I tend to be serious and precise in dialogues...
Well, it all depends on the game