A person who is considered to be a defensive player is often the easiest player to beat. That is because, although the defensive player will always minimize his losses when the cards are running bad for him, he will also minimize his chances of winning under any normal circumstances.
The defensive player is one who has a tendency to knock whenever he has the opportunity. Rarely does it matter to him whether or not he should go for gin, or whether he should try to underknock, it only matters that they end the game with a winning score. Not to say that this doesn’t work all the time, but the count will probably be minimal when they win so it will essentially take them much longer to win a game. This also gives the aggressive player a chance to catch up by getting a high score in any given hand.
A defensive player does have a chance against the aggressive player in the way that if he continues to knock while the opponent is constantly trying for gin, it will inevitably throw off the opponent, making him play worse in most cases. Also, it will force him to alter his style of play to one that he may not be all that familiar with, and in the long run this will prove to be an advantage for you.
Since the defensive player is more of a consistent knocker, he should be played n a somewhat more defensive matter. More consideration should be give to holding cards to lay of on him, forcing an underknock. However to the defensive player, a few underknocks in a row will discourage him from knocking too quickly and will force him into a game that will not be familiar to him.
A defensive player can also be rattled by the fact that you can figure out his game and may choose to pick up a few of his early discards, whether you need them or not. This disrupts his entire method of playing and puts him into a position of trying to defend against those various cards, until he finally loses all hopes of winning.
It is much more often that a defensive player will alert his or her opponent to their style of play early on, making it more difficult to win consistently. Defensive players also tend to discard relatively safe cards much to the chagrin of even an average player, but it can be used against them. If a player follows this defensive pattern, it is quite easy to figure out which safe cards he will eventually throw and the opponent should try to keep unmatched cards in his hand that will be helped out by the safe card that was discarded.
As you can see, there are advantages and disadvantages so the rule of thumb is to be defensive when needed, but not all of the time. When you are a strict defensive player it is much easier for your opponent to catch on and use this against you.