Ostensibly, Once Upon a Time in Mexico is about all the standard action-movie stuff drug cartels, an assassination plot, revenge killings. Been there, done that a thousand times. But really it's an exercise in style by Robert Rodriguez and not to be taken any more (or less) seriously than his giddy Spy Kids movies.
Sure, the body count is astronomical, but the picture is so cheeky and ironized that you come to think of all those extras flying through the air as a kind of corps de ballet, people you know are going to dust themselves off and head for the craft service table once the camera stops turning. Meantime, Rodriguez is more than likely choreographing some brilliant variation on a standard action trope. The high point here is El Mariachi (Antonio Banderas) and his bride (Salma Hayek), chained together, in a breathtaking escape down the sheer wall of a hotel. It is beautifully managed a cliched situation without any cliches in its development. Rodriguez even stages an eerie gun battle, in which all the weapons are silencer equipped. Wow, you say, why didn't anyone think of that before?
For that matter, why didn't anyone think of playing a rogue CIA agent the way Johnny Depp does? He's polite, dry witted, a bit of a gourmet, but also a master schemer and a psycho who thinks nothing of offing a chef who has offended him. It's an alarmingly funny performance by a fabulously daring actor, and it sets the tone for a movie that is not to be taken seriously except as a bedazzling step forward in the admittedly amoral aesthetics of violence.