Auto B-Roll: Add Stock Footage From Your Captions

Quick answer: In Zella’s editor, add captions first, then open the AI panel and click Add B-Roll. Zella reads your captions, picks the most visual keyword per line using on-device text analysis, searches Pexels for matching free stock footage, and drops short full-frame, silent, ~4-second cutaways across your video — spaced out, not every N seconds. You need a free Pexels API key (get one at pexels.com/api); Zella asks for it once and keeps it only in the macOS Keychain. Only the keyword leaves your Mac — never your video.

On this page: what it is · one-time Pexels key · add captions first · run it · what it places · swap, trim, remove · privacy · FAQ

What is Auto B-Roll?

B-roll is the extra footage you cut to while someone keeps talking — a shot of a city, a laptop, a coffee cup — instead of staring at the same face the whole time. It makes a video feel edited and keeps people watching.

Auto B-Roll finds and places that footage for you. It reads your captions (the written transcript of what you said), figures out the most visual word in each line, finds a matching free stock clip online, and lays it onto your timeline as a short cutaway. What used to be a manual chore — pause, search a stock site, download, drag, trim, repeat — becomes one click.

It lives in two places in the editor:

The AI panel has an Add B-Roll button.
The AI Assistant can do it when you ask, for example: “add b-roll based on my captions.”

How to set up your free Pexels key

Auto B-Roll gets its footage from Pexels, a library of free stock video. To search it, Zella needs a Pexels API key (a free personal access code).

Go to pexels.com/api and sign up (it’s free).
Copy your API key.
Back in Zella, run Auto B-Roll. The first time, Zella prompts you once to paste the key.
Paste it in. Zella stores it only in the macOS Keychain — the same secure vault your Mac uses for passwords — so you never have to enter it again.

Pexels clips are free to use, including commercially, and usually need no attribution. There’s no extra cost from Zella and no subscription for this feature.

Step 1: Add captions first

Auto B-Roll works from your captions, so you must have captions before you run it. If you skip this, there’s nothing for it to read.

In the editor, open the Captions tab and run Enable Captions / Transcribe audio (see the captions guide), or
Ask the AI Assistant to add captions first.

Once your video has a word-level transcript, you’re ready.

Step 2: Run Auto B-Roll

Pick whichever way you prefer:

Button: open the AI panel and click Add B-Roll.
Assistant: type something like “add b-roll based on my captions.”

Zella then, on its own:

Reads each caption line.
Picks the most visual keyword in that line using on-device NaturalLanguage — Apple’s built-in, on-your-Mac text analysis. Nothing but the derived keyword ever leaves your computer.
Searches Pexels for a matching short stock clip.
Downloads the clip and places it on the timeline as a cutaway.

The clips are spaced out across your whole video, not dropped blindly every few seconds, so the b-roll feels intentional instead of mechanical.

This step needs the internet — Zella is fetching stock clips from Pexels. It’s the one clearly network-based part of Zella; your own recording and footage stay on your Mac.

What Auto B-Roll places

Each cutaway Zella adds is:

Full-frame — it fills the screen, covering your main footage for the length of the clip (a true cutaway, not a small picture-in-picture).
Silent — the stock clip’s own audio is off, so your narration keeps playing underneath.
About 4 seconds long — short enough to keep the pace moving.
Spaced across the video — distributed over the whole timeline, tied to the caption lines with the most visual keywords.

Everything lands as a normal overlay on the timeline, so you stay in full control of it.

How to swap, trim, or remove a clip

A placed b-roll clip behaves like any other overlay — nothing is locked in:

Swap it: don’t like the clip Zella chose? Remove it and drop a different one in, or add your own footage in the same spot.
Trim it: drag the edges on the timeline to make the cutaway shorter or longer.
Move it: slide it earlier or later so it lands on the exact word you want.
Remove it: delete it like any overlay, and your main footage plays through underneath as normal.

Run Auto B-Roll for a fast first pass, then tidy up the handful of clips you want to change.

Is Auto B-Roll private?

Auto B-Roll is opt-in and clearly network-based, and it’s careful about what it sends:

Only the keyword leaves your Mac. Zella derives a single visual word from each caption line, on-device, and sends that word to Pexels to search. Your video is never uploaded.
Keyword extraction is on-device. The NaturalLanguage step that picks the word runs entirely on your Mac.
Recording and your own footage stay 100% on-device. Only the b-roll search and download use the internet — because fetching free stock clips has to.
Your Pexels key is stored only in the macOS Keychain, never in a plain file.

If you’d rather not fetch anything from the internet, simply don’t run Auto B-Roll — the rest of Zella keeps working entirely on-device.

Auto B-Roll FAQ

Do I need captions before I can add b-roll? Yes. Auto B-Roll reads your captions to pick keywords, so run Enable Captions (or ask the Assistant to add captions) first.

Where do the clips come from, and are they free? From Pexels, a free stock-video library. The clips are free to use, including commercially, and usually need no attribution.

Do I have to pay for the Pexels key? No. A Pexels API key is free — sign up at pexels.com/api, copy the key, and paste it into Zella once.

Does my video get uploaded anywhere? No. Only the keyword derived from each caption line is sent to Pexels to search. Your video never leaves your Mac.

How long are the b-roll clips, and do they have sound? They’re about 4 seconds each, full-frame, and silent, so your narration keeps playing underneath.

Can I change the clips Zella picked? Yes. Each clip is a normal overlay — swap it, trim it, move it, or delete it like any other.

Why is b-roll the one internet feature? Because fetching stock footage requires the internet. Recording and editing your own footage stay 100% on-device; Auto B-Roll is opt-in.

Pro tips & gotchas

Captions first, always. No captions means no keywords to search, so Auto B-Roll has nothing to work with.
Enter the Pexels key once. After the first prompt it’s saved in the Keychain; you won’t be asked again.
Treat it as a first pass. Let Auto B-Roll place clips across the video, then swap or trim the few you want to change.
You need to be online for this step only — it’s fetching clips from Pexels.