...and Double Blinders; The Players Don't(Stand Alone).

This post is a direct continuation of this post, unlike most posts which build indirectly on what has come before(Yes, reading this blog in order helps greatly). Seems like there'll also be a surprise third post in this series, though I'm unsure if it'll be next or one after, since the next two posts build on one another!

So, when playtesting it's important to test the mechanics on their own, to see how they work, rather than how your interpretation of them makes them work.

So, the players you play with will dictate a lot on how the game goes, you play the game, but so do they. When you play a game for fun you'd rather play a game with people you know, so you can gel as much as possible with them and collaborate to create a fun enviroment.
But what about when you play a competitive game, and especially when playtesting one?

I'll begin with several stories, taken directly from the First Post on this blog:
"..(In M:tG)Another instance was of me creating a deck specifically for multiplayer. But rather than have a deck which hurts multiple opponents to my benefit, I've created a deck based on "Fear Factor". The deck made use of Pestilence, a card which hurts all players and characters equally. I told the others so: "You don't attack me, I may or may not use it, you attack me, I use it and everyone, including you, suffers". It worked, no one wanted to lose, so no one attacked me, letting me watch with glee and mess with everyone as I saw fit. This was an especially good choice for me as I am often the "Strong Pick" and thus marked for execution early on, more on this later...

...Enter Settlers of Cattan, a classic if I ever saw one. I've first played Settlers of Cattan in a convention, where the three other players knew one another and I knew none. You may think that I had the disadvantage, that they will unite against the unknown, the outsider, and will only later turn against one another to finish business. That was not so.
People who know one another mark each other as "Strong" and "Weak", "Ally" or not. They assume that the opponent they know to be strong must be stronger than the unknown. So I used it to my advantage, as two of the friends united I offered an alliance to the last remaining player, and dumped him the moment I got what I wanted from him, trusting in my own capabilities. His friends later would not ally with him for he allied with me, leaving him alone, as I was, but much weaker.
Towards the end the other players noticed my burgeoning kingdom and decided to ally together in order to stop me. This was too little and far too late. Their pooled resources could not stop me..."

I believe you should play with people you know, whereas Dave Michael, author of Legends of Lanasia, believes otherwise. Dave, I'm inviting you here to explain in the comments why you believe you should play(test) (competitive?) games with people you don't know.

When you play with people you know who is a danger, you know why they are a danger, you know who will betray you.
I play Worms with my cousin against two computer enemies, we always agree to squish them first and only then turn on one another, but when that "Sweet Hit" calls, and since no one wants to get hit first we turn on one another, but we always know it'd come...

Sure, you don't fully test the system, but the system only builds example conflicts, situations for you to clash. There must be some conflict between the active agents in order for conflict to actually occur. This is often done by human nature, expectation and the need to prove oneself's as winner.

Sure, you know what others plan to do and what are their weak spots, but that doesn't mean you won't fall again, like Charlie Brown and the Football, or me and my cousin. You know the others' weak spots, but so do they know yours.

It is considered "Bad Sports" to bring outside occurances to the game, but let's be frank, much of the conflict inherent to competitive games actually comes from the friction between the players. Why create artificial conflict when there's real conflict to be drawn upon so easily?

You want to test the system, you don't want to create conflict between players. You want to test the game, you do not want to test the situation between players, just use it as a tool.
You have winners and losers, and those feelings add on to the next time you play.

I believe you should play competitive games with people you know.