I'll start with how it would be easy to look at this performance which may seem just like the very best of John Candy doing what he did for most of his unfortunately brief acting career. Candy on the surface at least goes for the entertaining performance as the man who just might have too much personality as he helps Martin's Neal try to make it home no matter what. In this way Candy is at his very best in this style of performance as the man who tries to do whatever is best but too often screws up and ends up having too many behavioral manners that might be just a little obnoxious.
Candy brings out his charm very well here and is very likable in the role of Del even though he doesn't shy from allowing us to see why Neal would have such strong outbursts against him at times. Candy mediates within the role very effectively finding the right balance in bringing the humor needed for the part through his excellent timing yet never overplaying it to the point that it makes Del just a caricature. Del is a character at the risk of being too much at every moment but Candy through his charm ground Del in just the right making the bigness of Del's personality a natural trait within the character that really works splendidly throughout the course of the film.
This film is of course a buddy picture and the film is all about the interactions between Candy and Martin as Del and Neal try to make it back to Chicago in time. Again on the comedic side of things the two work splendidly together as they play off each others styles quite effectively with Martin's cynical rather blunt performance which contrasts well against Candy's much more jovial and optimistic portrayal. They make a great balancing act and build to the punch lines often well with Candy's usually building something up in a colorful lively fashion which Martin without a doubt will break down with just the right severity.
Candy's work here though is more than just a nice enjoyable little turn to lead this road picture. Del's wish to help Neal to whatever end is not just a random trait actually though and Candy's performance actually gives motivation to Del. Del wants to be Neal's best friend and sees a camaraderie as two men on the road who are not where they want to be but their major difference comes in the secret that Del is hiding. Del claims to be happily married but the truth is he is a widower. What Candy does so well is that, even though this is a secret to the end of the film, he actually does give it away earlier by showing how this fact of Del's life actually shapes the man.
Candy's best scene probably of his whole acting career comes the first extremely rough patch in Del's and Neal's time together. Neal insults Del's whole personality leaving Del to defend himself. Candy is absolutely heartbreaking in this scene as he shows Del just barely able to hold back tears, but as well a passionate determination as he defends his genuine personality. In the moment Candy reflects the feeling of a past happiness with his extremely moving delivery of "I like me. My wife likes me.". Candy realizes the memories of Del's wife beautifully and suggest both the joy that the memories bring but as well the heartbreak over the loss as well.
In a way Del's reaction might seem like an overreaction but its not because of Candy's performance that explains why it is strong as it is. Del is not just being told he is a bad storyteller by Neal, but rather that there is a whole problem with his personality. Candy shows that it hurts Del deeply because Neal's insult is also a failure of his to actually reach out and find some happiness in a friendship while he has to live with the fact that he really has absolutely nobody in the world. Within it though Candy is terrific in bringing the resilience in Del who no matter what will stand by himself because he knows he is completely true to himself.
Martin and Candy are equally adept at the serious elements although for the most part they do make it an underlying factor in the film but one that slowly builds to a mutual respect to one another. Their chemistry is excellent and they naturally build the friendship even between the more comic moments. When the pivotal moment comes in which Neal comes back for Del, having figured out the secret that he had been hiding, it is heart warming without being forced in any way. Candy and Martin earn the moment completely through their performances.
John Candy's performance on the surface is just a very entertaining turn utilizing his great comic timing and his unique charm as an actor. This performance is more than just that, and that really would have been enough for the film to have worked out just fine anyway. Candy does more than that and reveals a side to his talent that tragically was underutilized during his career. Candy really brings a tenderness and emotional truth with his portrait of a grieving man who faces his grief, as well as the world, by trying to be the best man he can possibly be and try to find some happiness in that.