One can divide the kind of testing being done on the project into two distinct classes: guided and unguided. Guided testing customarily happens earlier in the project, when the game is not yet completely functional. In that period, the designer knows what portions of the game are clearly incomplete, but wants to get some feedback on a section of the game he thinks is working fairly well. Then the designer may direct the testers to try a particular level or section of gameplay. Directed testing may also occur later in the project, when the entire game is functioning but a particular section has just been changed or reworked. At this point the designer may need feedback on just that section, to see if the changes fix an existing problem or break the game in some major way. A designer may also direct his testers to try crazy and illogical ways of playing the game, to see if the game breaks under those circumstances. What may seem like a foolish way to play the game to you or even the tester may well be a style many players will try to use when playing the final product. More experienced testers know to keep trying every play style they can imagine when testing the game.
It is essential to allow and encourage your testers to do unguided testing as well. Give them the game, tell them to start playing it, observe what they do, and listen to their feedback. Many designers make the mistake of using only guided testing, usually having the testers test only the system on which they are currently working. When the testers bring up complaints about some other portion of the game, the designer will complain that he is not interested in working on that now, or that the problematic part of the game is already done. Directed testing has its place, but if it is all the designer ever does, then he is likely to miss larger problems in the game that he may not have even realized were problematic. Undirected testing gives the designer feedback about the game holistically, something that is essential to resolving all of its problems.
Of course, even when you do direct your testers to test only a certain section of your game, often they will not be able to resist pointing out the other problems they see along the way. It takes an extremely disciplined tester to truly test only the system that the designer requests . Getting feedback on parts of the game that you are not currently working on may be frustrating but can be useful in the long run. When testers give you off-topic suggestions about how to improve the game, even if you do not want to address those issues immediately, be careful to take note of them to come back to later. Nothing is more frustrating than recognizing a problem in the game after it has shipped, only to realize that one of your testers had told you about the problem in plenty of time to fix it.