Calling the current crop of three Indiana Jones films a film series is like calling the Gigget movies a film series. They may all have the same main character but the films only remotely connect.
In the first film, we are introduced to Indiana Jones, but we don't really get to know him until we meet him again later in the Last Crusade.
Although the first film may have been ground breaking, the last of three (until the fourth gets released) gives us the most depth, and hints at a stronger influence of director Steven Spielberg as we struggled through yet another fractured family.
Defining where George Lucas leaves off and Spielberg begins is difficult because both great film makers play with similar themes - as the relationship between Darth Vader and Luke Skywalker can attest. While the first movie remains my favorite, the third is the most interesting from a critical point of view, a Freudian field day right down to the sexual conflict over the Nazi mother figure and just which of the two Jones - father or son - gets to sleep with her.
Indiana - who turns out to be really named Harry, Jr. - evolves out of a conflict between the academic pursuit of knowledge and physical challenges of pursuing secrets in the field. While Indiana appears to follow in his father's footsteps to become a scholar, he adopts another identity from a more acceptable father figure he meets as a boy in the beginning of the film - an Indiana-like character who brokers rare objects not for study but to sell to wealthy clients.
The third film redefines Indiana Jones by painting him as a man who - while respecting his father by following his intellectual pursuits - has chosen another father figure to imitate, with both of these influences in constant conflict inside of him.
The Last Crusade, however, is less about resolving this conflict than it is about coming to terms with one of his father figures, proving his worth to his intellectual father who has always been remote, judgmental and dissatisfied with his son's achievements despite Indiana's near equal status. This third film in the series forced the father to come to terms with a son largely modeled after another man, to even move towards the physical world of his son.
Yet this resolution leaves room in some future film for Indiana to possibly come to terms with his other father figure - an aspect not fully exploited in Last Crusade.
The third film also is very rich in a more sophisticated social humor largely lacking in the first two films. We get a greater range of emotion, also not previously explored as the third movie breaks out of the comic book format the first films used. We get slapstick. We get pathos. Although in many ways the film's emotional landscape is still limited. After all this is still an action film, not a literary effort to explore the father-son relationship.
The real challenge in analyzing the Last Crusade is to determine which part is uniquely Lucas and which part uniquely Spielberg, and what parts were by mutual agreement. In films like War of the Worlds and Jurassic Park, Spielberg's influence is obvious since he has inserted his usual suburban trappings into the tale, and we only have to look at the original text to see what it is he added. These signature pieces also give away the huge influence Spielberg had on other pictures such as Gremlins, Back to the Future and Short Circuit which he did not direct.
But Spielberg and Lucas are so closely aligned in some themes that I find it difficult to divine who did what aside from the most superficial elements such as the continued use of smoke to make the woods scenes more mysterious.
I suspect Spielberg had the least influence over the story-line of Temple of Doom - except for the installation of a child side kick.
While both Spielberg and Lucas use children as vehicles in their work, they tend to use them in different ways. This is a bit of a generality but the difference seems similar to the difference in characterization of children in Tom Sawyer and Huck Finn by Mark Twain. Lucas - as with Twain's Tom Sawyer - seems dedicated to capture the essence of childhood. Spielberg, however, uses kids to influence adults, to evoke some change in them as in Jurassic Park and in War of the Worlds. The Ray Ferrier who begins War of the Worlds is much different from the one who ends it, partly due to the responsibility he takes on in helping his kids - at least one of them - to survive.
The influences of the two film makers are most obvious in the Last Crusade's opening sequence where Lucas was able to unveil a boy Indiana Jones with all his impishness and Spielberg was able to play off the uncomfortable relationship with his father.
The fact that Henry Sr. has to raise the boy alone as part of a fractured family is a theme Spielberg revisits again and again.