Back in 1975, in his contribution to The Wild Hunt, the late Glenn Blacow made the observation that what you were doing in D&D was playing a role. Other figures in the early community took up that idea, and sort of expanded on it, interpreting it to mean that Dungeons and Dragons and the like were what amounted to role playing games. Thing is, from what I can see the purpose of a game such as D&D is not to play a role, but to accomplish tasks. You can assume a role, but that's not really why you're there.
At the acting game link I've provided here you get a series of exercises for young children. Those exercises, those games, are designed to help teach children how to play a role. They will work for adults. That is the purpose of an acting game, to act. To play a role. You're not out to win anything, but only to become a better actor, a better role player.
In games like D&D you have numerous goals, but acting (role playing) isn't one of them. Any role playing you do is incidental and there are really no mechanics for them. Even the now defunct Theatrix really doesn't have mechanics for acting, and the only system I can think of with what you can call acting rules would be the mechanics for social play provided in an accessory by Bill Stoddard for GURPS. Even Mythus doesn't really have express role playing mechanics, those are more often suggested than concrete.
Why Role Playing Games?
That we are playing role playing games is really more an assumption than an actual fact. And now that I've gotten this far I realize that this is going to have to be a series. So for the next in this series we'll have a look at what acting (role playing) is.