Well, it depends on the game to how serious you should take it. Freaking out because of the goomba's in the original Super Mario Brothers getting you is just stupid. But playing Halo 2 with your friends, it just entertaining taking it seriously and rubbing victory or defeat in each others faces. And forum RPing, you need to take seriously to a point. If you take it seriously enough to get yourself to the point of truly feeling with your character, and of great RP then that's good...just don't start flamming people OOC and freaking out OOC when things go badly.
And my name has no 'D's in it what-so-ever

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