If you've been picked as the approver for a post, it will wait for your decision before it can publish - even past its scheduled time.
Where to review
Open the Approvals page - it lists every post waiting for your review. Each entry opens a review page showing the media, caption and scheduled time exactly as they will publish.
Your two options
- Approve - the post is cleared and will publish at its scheduled time (or immediately, if it was set to publish right away).
- Request changes - write a short note about what to fix. The note is attached to the post so the author sees exactly what you want changed. After they edit, the post comes back to you for approval again.
Nothing publishes behind your back: a post that requires approval and hasn't been approved simply stays put.