As this is a short story collection with a framing sequence to unite the different stories, there is no real character development worth noting.
This is a short story collection with a framing sequence. The framing sequence is meant to tie together these stories in a very loose fashion - with the exception of "The Illustrated Man", none of these stories have any apparent relation to one another or to the framing sequence. However, they tie together in ways outside of actual plot.
As for individual stories, Bradbury often lays out a situation and follows it to a conclusion that seems inevitable to the reader. The premise may be fantastic in some manner, but ultimately spring from the predictability of human nature. Sometimes he provides a plot twist to the end - Hernando is puzzled by the panic of the Americans in "The Highway for example, or Braling is replaced by his marionette in "Marionettes, Inc.".
However, these twists in the stories of The Illustrated Man don't often change the careful reader's perspective and understanding of the situation, which is a common enough effect with other short stories (especially in the horror and detective genres). Rather, they provide an unexpected variation on the inevitability of the situation: for example, it seems only fitting that Ettil Vrye is run over by a car of screaming teenagers, he was already a victim of Earthian subjugation. The only twists that re-evaluate the situation occur in "The Other Foot" and "The Fox and the Forest", where both stories deal with the oppressive aspects of a society and benefit thematically from playing the complexity of their plot twists.
Themes didn't develop in this collection of short stories; rather, when different stories handled the same themes, they often provided different perspectives on the same concerns. This in itself is instructive, if only because it shows the complexities of Bradbury's ideas and beliefs in his fiction.
The dangerous nature of the creative imagination manifests itself in many ways, but is almost always fatal. For daring to partake in the stories of the Illustrated Man, the narrator of the Prologue and Epilogue has unwittingly placed his life in danger. Ettil Vrye of "The Concrete Mixer" fears the Earthian imagination as a way to fend off a Martian invasion; however, even he could not predict how the Earthian imagination would debase and destroy the supposed conquerors of their planet. Censorship threatens the literary characters of "The Exiles," who try to fight back but are ultimately defeated by the forces of logic and rationality. However, faith can help elevate the power of imagination: Father Peregrine's memories of childhood and sense of awe when watching fireworks and fire balloons reflect the awe he feels about his religious faith and his encounter with Martians; this alarms Father Stone, who is more "adult" in wanting to stick to conventional views of religion, sin, and redemption.
That said, the imagination of children is the most dangerous in these stories, as seen in "The Veldt" and "Zero Hour". In those cases, as well as the space-centered imagination of Doug in "The Rocket Man," there are fatal consequences of one kind or another - though the children remain intact at the end. However, children's imagination has a more positive, redemptive nature in "The Rocket" and even the ending of "Kaleidoscope". So it isn't the child's imagination per se, but how it relates to the families of these children. Which leads to the related theme of family and its dissolution: in the stories where children have fatal imaginations, the family doesn't fully understand the needs of its different components - parents allow technology to care for the children in "The Veldt" while the age-old condescension by adults of child's play is at the heart of "Zero Hour".
This inability to bridge gaps and have more effective parenting proves the undoing of the parents, who are killed. In "The Rocket Man," Doug plays a more neutral role, wishing for the stars but ultimately providing solace to his grieving mother. However, there is a stronger sense of family unity for the children in "Kaleidoscope" and "The Rocket" - if anything, the imaginative experiences they undergo help build bonds between children and parents.
The abuse of technology is a frequent theme of Bradbury's and is evident in most every story in this collection in one way or the other - not a surprise, since this is ostensibly a collection of science fiction tales. In some stories, technology has taken over too many human roles - as in "The Veldt" and "Marionettes, Inc." - while in others it's simply beyond human comprehension - as in "The City" and "Zero Hour". However, technology also provides a means to escape a horrible situation ("The Fox and the Forest") and relief from an unforgiving natural force ("The Long Rain").
Related to this issue of science is that of belief: that is, the theme of faith or its lack in these stories. "The Man" shows what happens when someone has no faith in redemption, while "No Particular Night or Morning" shows the danger of extreme solipsism (the belief in only one's own experiences as the basis for judgments). This ties into the theme of remaining sane in an insane situation, as "No Particular Night or Morning" is also about enduring the rigors of space travel - Hitchcock's solipsism springs from an inability to endure such a situation (or to actually welcome the insanity of being surrounded by nothingness) while several of the men in "The Long Rain" give in to the slow torture of the Venusian weather. However, the lieutenant in that last story has faith that he will reach the haven of the Sun Dome and is rewarded for this belief. Similarly, faith in a higher power does save Martin in "The Man" and provides a new way of seeing spiritual redemption in "The Fire Balloons", one tied closely to the distinction between mind and body.
Yet another theme related to this nexus of concerns is the acceptance of death. With his strong faith, Father Peregrine actively challenges death and defies it. That said, three other stories deal with actually dying in outer space: "Kaleidoscope," "The Rocket Man," and "No Particular Night or Morning". Interestingly, the promise of a quick death in "The Rocket Man" is contradicted by the prolonged experience of Hollis and the crew of "Kaleidoscope". However, these astronauts all seem to accept their deaths - Hollis uses it as a way to make peace with his unsatisfying life, while Dad in "The Rocket Man" has made his peace with such a death before his final flight kills him. Most disturbing is Hitchcock in "No Particular Night or Morning," who seems to actively court the nothingness of space as a step towards his own demise.
However, his may not be the only death wish in the book: "The Last Night of the World" has its domestic couple reason that the quiet routine of their lives led to a neutrality that allowed evil to flourish elsewhere in the world, thus "earning" the death of all humans. This actually ties into fears about mechanization and the deadening routine of everyday life, as seen in "The Veldt" and "The Concrete Mixer" - a more existential variant on the abuse of technology.
Revenge is the theme of several stories, but doesn't prove satisfying. The most vivid example is "The Other Foot," which also touches on issues of racial tensions - and while Willie Johnson has seemed to earn his right to lash back, he takes the higher moral path by denying this base desire. Similarly, the title construct in "The City" has its revenge but that only allows it to die afterwards as it had no other purpose. The children of "The Veldt" have revenge on their parents' threats to shut down their nursery, but this is a chilling vengeance - as is Mink's promises to kill various groups, including older kids and her parents, in "Zero Hour". The one story where revenge would be welcome by the readers - the revolt of literary creators and creations in "The Exiles" - proves futile, as they are wiped away by a last book burning. (This may be contrasted to the satisfying - if morally questionable - revenge in another anti-censorship story, "Usher II" in The Martian Chronicles.)
Last, there are the themes of warfare and social strife, reflecting Bradbury's desire for his stories to be potent social commentary. War is portrayed strictly as folly, seen in a wide swath of stories, including "The Highway," "The Fox and the Forest," "The Concrete Mixer," and "The City". But each takes a different tack on the consequences of war: in "The Highway," the perspective of Hernando is quite different from that of the Americans - his world is simple and is not threatened by atomic war in the same manner. After all, being blown back to the stone age doesn't matter if one's only a few steps above that age in the first place. In "The Fox and the Forest," the focus is on the persistence of fascist regimes to claim what is theirs; that is, it provides a cautionary tale, but one with no hope for those who wish for freedom. "The Concrete Mixer" and its satire of cultural imperialism is similarly bleak, as the corrupting influence of American culture manages to poison the Martian invaders in very quick order, making them more Earthian than Martian - or killing them in the process. "The City" shows how long-standing enmities can hold unexpected consequences, as the City apparently commits homicide / suicide by destroying Earth before itself dying. Other themes of social concerns aren't handled as thoroughly - racism only plays a major role in "The Other Foot" (which contrasts to the welcoming of an entirely alien race, the Martians, in "The Concrete Mixer") and censorship in "The Exiles".