In gathering the panels for this project, certain visual patterns began to emerge. This attempt to describe some of them is by no means definitive nor complete.
Upper-case letters are preferred in comics, and typically get written larger than the surrounding text when someone is in pain. There do seem to be exceptions to this (see examples to left), when the pain is very sudden or quickly overwhelming. In the second example, the letters are made to appear smaller by enlarging the space between the letters and the edge of the word balloon.
Otherwise, there are a lot of standard techniques to use on lettering; making
it look harsh in shape, or thicker, or italicized, etc.:
The whole point is to depict emotions resulting from physical sensations, through the use of shape. However, additional information can be shown through relative positioning:
Changes in lettering size can show changes in volume or emphasis.
In this panel, the slant of the letters was used to indicate that someone was falling.
But the most common technique I noticed was the use of a zig-zag pattern, which seemed to indicate unsteadiness in someone's voice, or a loss of bodily control.
Underlining is a common way to emphasize a word, while drawing a line
THROUGH the text is rarer. Tick-marks are sometimes added
around words: the "--URK--" panel above is one such example, and another very
common icon pair can be seen to the left. These should not be confused with
certain types of <surrounding brackets>, used to indicate that a
foreign language is being spoken.
Word balloons, like lettering, can convey additional information by shape.
Lettering is not necessarily confined to this space; sometimes it goes over the edges
or breaks out entirely. However, if words are depicted without a balloon, they should
be distinguished from other non-verbal, environmental sounds (wham, pow, smash, etc.).
The spelling of sounds is partially determined by (1) the conventions that comics artists have developed over the years, and (2) language and culture. All of the examples here are in English, but the way sounds are spelt changes in other countries and languages. (Addendum: the "Wurrrg" panel in my collection might be German for "Choke".)
For example, in English comics, the sound of a gun being fired is often shown as "BANG" or "BLAM", but in French, it becomes "PAN" - pronounced a bit like "paw", but more nasally towards the end. The closest English equivalent would be "POW", which is often used when someone is punched.
The first spelling pattern to emerge from this project was that discomfort words most commonly begin with A, O, and U - all are vowels; slightly less common are G, M, N, W, and sometimes Y.
"A" sounds are often uttered when someone is actively observing their own trauma, either while or just before it happens. Typical situations are those of pain, fear/surprise, and falling. It is sometimes followed by other vowels such as I, E, or a silent H. G is very popular to combine with A, and G is often paired with other consonants such as H, R, and K.
"M" sounds happen when someone's mouth is blocked. The letter M is usually repeated a couple of times, followed by an F-like sound.
"N" sounds are for pain that is internal and/or specific - headaches, torn ligaments, etc. Often paired with G or GH, and the occasional vowel.
"O" sounds fall into three general types. "Oh", "Oof", and "Ow" (or its longer variant, "Ouch"). "Oh" doesn't seem to get used that often, and its use is vague. "Ow" is for minor forms of discomfort, nothing extreme. "Oof" is used to indicate collision or impact, a sudden unbidden exhalation of breath.
"W" and "Y" sounds are often those of surprise, questioning, or loud yells. "Whoa!", "Wha?", "Yeow!" and so on.
"U" sounds, like "Oof", seem to indicate the pain of sudden impact, but I get the impression that the point of impact is a bit more localized and sharp. U gets joined with a whole bunch of consonants: G, H, K, L, N, R; quite often as GN, GH, HN, or NGH. To my surprise, I didn't find a single "Unh!" in the course of this project - maybe it's too simple? "Unhhh..." was the closest match.
Once you've chosen your vowels and consonants, the next decision is to determine length and emphasis. How often do you want to repeat a letter, how complex should the exclamation be? The most complex ones I found were "Akgkh!", "Gnnghh!", and "...Uhhnhgh!!!". Note that punctuation, like letters, can also be repeated for emphasis.
Finally (although I don't have any visual examples), letters themselves can be abandoned completely. Combinations of "?", "!", and "..." can indicate a variety of pauses and emotional states. Expletives can get expressed with typographical symbols ("$@&!") or other icons - skulls, stormclouds, or chaotic back-and-forth pen-scribbles. Basically, the art of comics has developed a rich language with which to express itself.