For our purposes acting is the art of pretending to be somebody or something other than what you are. In the realm of RPGs that is supposed to include behaving in such a manner as to direct others into thinking of you as that "other", and just as importantly yourself.
With some it comes naturally, others don't even bother to try. Then too there is the fact that not everybody sees the role you play the way you do. You have the savage as presented in old adventure stories, and then you have the savage as presented in the pages of National Geographic. By received wisdom the North American raccoon is a wild animal that studiously avoids humans, which according to videos one can see on-line are kept as pets that can get down right affectionate.
And let me add that one person's setting could have, let's say, illithids as conniving, treacherous, manipulative, brain devouring bastards. While another sees them as wandering merchants fostering trade and communication among the cultures they visit. Which means that often the player will need to adapt and adjust as he plays his role.
Just as important is the fact that in each group each member will need to be flexible in how he approaches his own play, the play of others, and the play of the guide. For the Dwarfs of the Viridian Glens are not the Dwarfs of the Fetid Wastes. My orcs are not going to be yours.
So being open and able to accept differences is important. Accepting that another is going to see matters differently from you is especially important. For such acceptance can and most often will mean more open play. I mean, treating all goblins as crazed killers gets boring after a while. Having to actually learn about who you've just met can be a lot more fun.
But the really important thing is to learn how to act. Act effectively, act convincingly, for it is your task to convince your fellow players that 4'2" tall Anne is now playing a 12' tall argent giant who can easily see over that 10' wall, and even help her friends get over it.
And why is acting so important? We'll look at that in part three.