Hey vanvid, welcome to the RPPR forums!
I think you may have read my comment on the "The Haunting" actual play; sorry for that, as you can see it was completely sarcastic (much like the actual description to the Actual Play.) The game was the first time Aaron (one of the RPPR cast-members) GM'd a game of Call of Cthulhu. This was in preparation of running a game at Gencon 2010, so the other players wanted to act as 'bad players' in case any of the unknown players at the Con game were bad players as well. Unfortunately for Aaron, he was late for the game, which meant that he had no control over the character generation of the players so they went wild and created the most "asshole" PC's they could.
So yeah, it was a joke, (Posted on the site on April 1st, no less.) which toughened up Aaron enough for his Gencon game. It's not really a good session to listen to for the first time, since normally the sessions are better in every way, shape and form.
Regarding planning for games, it really depends on the type of GM you are. There are many who don't write ANYTHING for their scenarios, and sort of make it up on the fly. There are others who will pour out their hearts and souls on writing pages upon pages for the scenarios. It all really comes down to how comfortable it makes you to have those notes to fall back upon, and how much faith you have on your creativity when put under the spotlight.
I myself spend 2-4 hours working on my scenarios, though my game sessions normally last 3-4 hours. Although normally I spend a whole week just thinking of stuff and writing it down somewhere, then spending a few hours writing the scenario based on those ideas.
I think it mostly takes practice. Getting the grasp of writing scenarios helps a lot to cut down the work-load. Knowing your group, how fast they go through content, how likely it is they will talk to an NPC, what their likely actions will likely be, what they like to do, etc. will also help you narrow things down.
You don't need to stat out every single character or give them all a unique backstory. You also don't need to detail every single room, just write down basic thigns they might see, and jot down details that might be important. For Masks of Nyarlathotep you have the added benefit that someone has already done a lot of the work for you, use that!
Hope that helps.