I'm going to do a three part discussion of playtesting. This is part one, about playtesting solo: i.e. playing all the players in a multi-player game. I'll do the next two parts over the next couple of weeks. I spent most of Friday solo-ing Jorvik, and with our impending move down South it's going to take me a while to set up a new playtest group, so I'll need to do a fair amount of solo playtesting over the next few months.
Solo playtesting is the first stage but also comes in later on too. At the beginning it allows you try out those ideas in your head that sound awesome, but when you try them just don't work. At all. Rather than testing it on real people, trying your friends' patience or getting the game a bad reputation with the public before it's off the board, you can use solo-testing to iron out the worst of the wrinkles before you get the game out in front of real people.
Solo-testing has its pros and cons like anything else. If you know what they are, you can use it more effectively. Solo-testing allows you to try out the game between opponents of the same calibre (they're all you after all!), to find any bias for going first or last. It allows you to try out different strategies to determine if a player using one strategy has an advantage. It allows you to quickly iterate through various ideas. Game not working? Stop it halfway through, change a few things a try again. Real people don't like that :-)
Similarly it has it's weaknesses too. Games with any hidden information (i.e. cards in a player's hands that only they can see) will be spoilt. You'll know what's in everyone's hands. You'll have to try and ignore that, pretending you don't know and instead play the odds: 'There's two floods in the thirty card deck. I've got four cards and no floods, so the odds that my opponent has at least one are (24C3 + 24C2)/26C4 = 15.4%. That's a risk I can live with, I'll do X.' It's hard to be objective about a game you're heavily invested in. You can't make a go/no go decision based purely on solo-playtesting - you need to see what others make of the game, and what flaws they find that you've missed. The more people you put the game in front of, the more likely you are to get a realistic impression of the game, and the more likely you are to find any flaws.