When i first saw the poster or trailer for this film I really expected it to be the usual silly Black film and in many regards that is what this is, but it also has some real acting in it that kind of shows what Jack is capable of.
I was also suprised to find out that this film is loosely based (and i suspect VERY loosely based) on a real story of Fray Tormenta, a real-life Mexican priest who funded his orphanage with money raised from being a luchador (masked wrestler in Mexico.)
Of course, part of the fun is the fact that Jack Black is not in great shape and he doesn't try to pretend that he is and I think a big part of his fame revolves around the fact that he is completely willing to make fun of the fact that his body is a little less than muscular. In the film, he does try to show off a bit after he experiences some success.
The matches he is in are absurd, but we would expect nothing less. Wrestling is actually taken very seriously in Mexico and has for quite some time. Some would argue that it is taken seriously in USA (well, in the world) via WWE. However, for a luchador, one of the most disgraceful things that can happen is for one's mask to be removed, and their true identity to be revealed.
It definitely is silly, but it isn't a full on comedy (if that makes any sense.) This actually made a lot of the critics respond negatively to it, as if they don't want black to experiment at all. I dunno. I liked it a lot more than the general public or professional critics seemed to but i will still admit that it isn't exceptional - but to be fair i don't think that any of Black's work, aside from Pick of Destiny, is.
