Why productions make them
Camera originals are big. A day of 4K raw from two cameras can fill several terabytes, and playing multiple streams of it on an editor's workstation stutters. Proxies let the editor scrub, trim, and play multicam without dropped frames, and they fit on a portable drive the director can take home.
The arithmetic is worth doing in prep. Say the A and B cameras each roll 2 hours a day across a 20-day shoot, which is 80 hours of footage. If you assume 300 GB an hour for the camera originals, that comes to 24 TB before backups. The same 80 hours as 1080p proxies at an assumed 20 GB an hour is 1.6 TB. Those per-hour numbers vary widely by camera, codec, and resolution, so check the manufacturer's data rate table for your actual settings before you order drives. Your shooting ratio drives the whole calculation.
Who makes them and when
On most shows, proxies are created as part of dailies. The DIT on set or a near-set dailies lab offloads cards, verifies checksums, applies the show LUT and any on-set color notes, syncs production audio, and renders the proxies overnight. The assistant editor ingests them before the editor arrives.
A clean proxy keeps three things identical to its original: the clip name, the source timecode, and the reel or tape name. A typical burn-in window on a proxy shows all of them, for example A042C003 | 14:22:10:05 | SC 42A-3, so anyone watching a cut can trace a shot to the camera card.
Where proxy workflows break
The problem nearly always shows up at the conform. If a transcode renamed clips (adding "_proxy" to the file name, for example) or dropped the reel name metadata, the EDL no longer matches the camera files and the online editor has to relink shots by hand. Frame rate mismatches cause trouble too. A 23.976 original transcoded to a 24 proxy will drift against sound and conform to the wrong frames.
Another mistake is treating the proxy look as the grade. The LUT on proxies is a starting point from the director of photography, and directors who spend months watching it can resist the colorist's work. Remind everyone early that color grading starts from the camera originals.
How proxies fit the post schedule
Proxy specs belong in the post workflow document that the post supervisor, DIT, and editor agree on before day one: codec, resolution, LUT, burn-ins, naming, and delivery time each morning. Drive purchases for originals, proxies, and backups sit in the post budget of your film budget, and the proxy naming plan is what makes the eventual conform an afternoon's work instead of a week's.