It's been awhile, but here's something to chew on.
There are a few ways to indicate a character's interior thoughts and feelings in the comics medium. They can remain silent, expressed entirely through the way an artist chooses to portray a character. In that case all one would have to go on is dialogue, and facial/bodily expressions. Another way of showing a character's interior life is through narrative captioning. In this form, thoughts are expressed directly in text narrated by a character itself or perhaps by some unseen omniscient narrator. This method has gained great popularity in superhero comics over the last twenty to thirty years. There is however one very classic method of depicting a character's thoughts that seems to have fallen out of favor as narrative captions have come to dominate: thought balloons.
In fact thought balloons were the predominate form of depicting a character's internal life up until about the early to mid eighties. What changed? As with most things in comics in the last two decades it boils down to the two 'M's. Misters Moore and Miller. In Watchmen Moore does away with thought balloons entirely. Instead we get Rorschach's journal. (As a side not I think this is the main contributor to reader's identifying with Rorschach more than Moore claims he intended. Moore underestimated the audience's ability to identify with a character when given an internal monologue. Rorschach's directness in the journal undercuts Moore's intended irony.) Miller's contribution is, of course, The Dark Knight Returns. In Dark Knight Miller makes extensive use of captions for most of the characters. Batman, Joker, Superman, and even Green Arrow all get little captions rather than thought balloons. Since Watchmen and Dark Knight Returns are often seen as ground zero for "BAM! POW! Comics are growing up", it stands to reason that their mutual abandonment of the thought balloon was seen as part of this growing up process. This was the starting point of superhero comics deciding that the way to be mature was to make the characters as hard and full of badassitude as possible. In come orderly square captions, out go fluffy puffy thought balloons.
However, there is one character, oddly enough created during the same period as Watchmen/Dark Knight Returns, whose comics not only didn't eschew thought balloons, but makes extensive use of them to this day.
Paul Chadwick's Concrete makes extensive use of the main character's internal world through thought balloons, but also uses captions featuring some unknown omniscient third person narrator, as demonstrated in this page from a fairly recent Concrete story:
Of course Concrete also lack some of the other hallmarks of traditional 'mature' superhero comics, like masked broody guys beating the shit out of people. Some debate remains whether Concrete counts as a superhero comic or is more of a sort of science fiction comic. I tend to fall on the superhero side although he's a very different sort of superhero. Perhaps, a sort of superhero comics could do with more of. A gentle, interior creature who uses his enhanced abilities for exploration and advocacy of various sorts.
Thought balloons allow the reader dip in and out of a character's interior reactions without the structural necessities that come with using captions for a internal narration. Caption narration turns the main character into the narrator of the story as well as the main character, and that's not necessarily desirable depending on the type of story being told. I would definitely welcome a return of thought balloons to stories featuring gentle stories centered around figures of great power. Character's like Superman and Captain Marvel come to mind as immediate examples of character's that might benefit from this approach.
The other place where such a device could prove useful is with character's such as Martian Manhunter and Professor where thought is an integral parts of the character's powers and skills. With such characters thought balloons could be used in conjunction with speech balloons to show a character saying one thing while thinking another. This puts us in the same position as the character always aware of duplicity on the parts of others. In fact, telepathy might be the one place where a caption box would just be too jarring since telepathy is direct communication from one character to another, like speech. Speech balloons do have their utilities and a better understanding of these utilities would, one hopes, contribute to better comics all around.
Next: More thoughts on thought when we discuss Professor X-savior!