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There are players who will try to play solidly when they first sit down, and continue to play solidly if they start off winning. But some players can change their styles drastically if they start to lose, particularly if they take a bad beat. All of a sudden, these players will go from playing solid to being a bit more aggressive. Then they will semi-bluff and bluff more. If they continue to lose and get some of their bluffs called, or worse, they get more bad beats, then they could wind up opening up their game and could turn into a maniac-type of player.
This is a situation where having played against the player in the past and knowing this particular characteristic is very useful. If it is the first time you ever played with him and he plays like this, you may assume this is how he plays and use that information the second time you play at the same table with him. But the second time around, he may be running good and playing solidly, in which case, your perception of him will be completely off.

Professional players are sharp and are constantly thinking about the game and their opponents. They are the best players and the ones that you least like to see at your table. They could be playing poker as their main source of income or they could be playing it as a supplemental income. They are playing poker to win unlike other players who may be playing for the gamble, the entertainment or just the competition. Professional players understand the importance of playing aggressively when they have a solid hand and they also understand the importance of choosing their hands selectively. They will try to extract the most amount of edge from their opponents when they have the best of it, but try to get away cheaply when they have the worst of it. Professional players have thought about and studied the game, be it through reading, talking to other players or just thinking about the play of hands on their own. Professional players will still have a wide range of characteristics in their style. Some will be more aggressive than others, some will bluff a bit more than others. Of course, there are always those who consider themselves as professional players who really are not.