You've seen them on every short-form feed: the caption sits on screen and each word lights up exactly as it's spoken. Karaoke captions, animated captions, word-by-word captions — same thing.
They're not decoration. On a muted feed, the moving highlight tells the eye where in the line the speaker is, which keeps a viewer reading instead of scrolling.
How it works
Ordinary subtitles have one start and one end per cue. For word-by-word captions the renderer needs a moment for every word, so it knows when to move the highlight.
OpenSubs works those out from the cue itself: each cue has a start and an end, and the words in it are spread across that span. That is why the effect also works on a subtitle file you imported — it needs nothing more than the timings the file already has. Storing the result takes a format that can carry per-word timing, which is why this kind of caption is rendered from ASS files, or drawn straight into the picture, rather than from plain SRT — see subtitle file formats.
When to use them — and when not to
Use them for short-form video, talking-head clips, ads, anything watched on a phone with the sound off.
Skip them for long-form video, lectures, interviews and documentary work. A highlight that never stops moving is tiring over ten minutes, and accessibility guidance favours steady, readable text.
Making them from your own video
In OpenSubs, free and in the browser:
- Open opensubs.app and drop in your video. Nothing is uploaded.
- Click Generate from the audio.
- Read through and fix any wrong words — the highlight makes mistakes more noticeable, not less.
- Under Word effects, choose Highlight each word as it is spoken.
- Pick a caption style. The heavy, high-contrast presets suit this effect best.
- Preview it over your own footage, then click Burn subtitles into the video and save the MP4.
There's a second effect in the same place — Size every word by how loud it was — which grows the words that were emphasised. Because it needs the audio, it's only offered for subtitles this app transcribed; highlighting works on imported subtitles too.
Getting them to look right
- Keep cues short. A few words at a time. A long line with one word lit up is hard to follow.
- Contrast first. The highlight colour has to beat the footage behind it; an outline or a solid box behind the text helps.
- Don't put the captions where the app's interface goes — the right-hand buttons and the bottom strip on Reels and TikTok.
- Preview on the real footage, not on a plain background. Busy shots are where captions fail.
Exporting
For social, burn the captions in: platforms like Instagram and TikTok don't take subtitle files, and the effect has to be in the picture anyway. See adding subtitles to an MP4 and subtitles for Reels and TikTok.
If you want a file rather than a burned-in video, export .ass: the style table travels with it, and renderers such as VLC and mpv will draw it. Open it in your player and check before you rely on it — how much of the word-by-word effect survives depends on the renderer.
FAQ
Do I need After Effects or a paid captions app?
No. The effect is standard subtitle rendering; a browser tab can do it.
Will it work on subtitles I already have?
Highlighting works with imported .srt / .vtt files. Sizing words by loudness needs the audio, so it's only available when the app transcribed the video itself.
Is there a watermark?
No, and no account either.
Does it work for languages other than English?
Yes, for the languages the model recognises. Long CJK lines are worth checking in the preview before you export.
Can I still turn the captions off later?
Not once they're burned in. Keep the subtitle file if you might need a clean version.
