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Do Japanese writers use underline to emphasize a point, or other techniques? Wikipedia mentions the use of katakana in its article on Emphasis and in its article on Katakana, but I'm not sure whether it's appropriate.
Background: I’m incorporating XKCD 386 (with the text modified) into a talk given in English with Japanese subtitles. I'm doing the cartoons side by side, and I'm wondering what the equivalent of underlining "wrong" in a western cartoon would be.
The Japanese equivalent of underlining for emphasis would probably be using 傍点【ぼうてん】 or 脇点【わきてん】:
Dots added over (if writing horizontally) or to the right (if vertically) of each character.
Wikipedia Japan has a page detailing their use, as well as their variants:
文字種としては、縦書きの場合は主に黒ゴマあるいは白ゴマが使用され、横書きの場合はビュレット（黒丸および白丸）が使用される。 上記以外にも二重丸や蛇の目などが使用されることもある。 傍点はゴマ、圏点は丸と区別することもあるが、区別しない場合も多い。
In substance:
If writing vertically, people mainly use the "black sesame (goma)":﹅ or "white sesame":﹆.
If writing horizontally, the round black (●) or white (○) dots are preferred.
The "good work" double-circle (◎) and bull's eye circle (◉) are also used.
While some people differentiate between "sesame" (goma) and round signs, many people don't and use them indifferently.
(there are a couple other particular cases, such as when reporting an erratum, but I don't think you are particularly interested).
To conclude, you probably want to use the simple round black marker, like such (with slightly less distance between the lines, typically where the furigana would be):
・・
よく聞いてくれ
Listen really well
