Best Boy
You thought:
- The producer's gimp.

Actually:
Responsible for running the rigging and lighting department, including crew hiring and schedules, liaising between the electrical crew and the other units on a film, booking rented lighting and grip equipment in and out, loading production trucks and setting up lighting. Term dates back to the apprentice system. Not always a boy.

The Gaffer
You thought:
- Vague authority figure
- Some old guy.

Actually:
Known sometimes as Chief Lighting Technician, the gaffer is the head of the electrical department and will oversee the construction of (and often design) practical lighting set-ups. Works closely with the director of photography. The term 'gaffer' is thought to derive from 'gaff poles' used to move a film studio's mirrored panels into usefully reflective positions back when movies could only be shot with available light. The need to provide adhesion by wrapping the poles with tape is one possible explanation for the term 'gaffer's tape'.

Key Grip
You thought:
- Some guy holding on tightly to scaffolding/camera equipment

Actually:
On US film crews, oversees the grips (those who rig lighting set-ups or operate camera cranes or dollies) on a union movie, co-ordinates the logistics of set construction with the gaffer (see above), supervises the laying of dolly track for cameras. European filming uses 'The British System', wherein the Key Grip is exclusively concerned with camera-mounting. Also responsible for on-set safety in US crews.

Swing gang
You thought:
- Morally flexible group of young people with similar tattoos.
- On-set Jazz entertainment.

Actually:
A bunch of guys who make last-minute set changes the director may ask for.

Clapper Loader
You thought:
- A guy dumbly trying to squeeze film into a clapperboard directly before being fired.

Actually:
Also known as second assistant camera, the clapper loader is (yes) the person you've seen topping and tailing blooper reels with the clapperboard, but is also in charge of loading film stock into camera magazines. It's actually a very well-paid job. And why not? Screw it up once and your movie career is probably over, particularly after failing to record a $4 million explosion. The only person on the crew who will handle or administrate the film stock at any point, the clapper loader also liaises with continuity regarding which takes get printed.

Line Producer
You thought:
- The guy who obtains cocaine for the executives and the primary cast.

Actually:
The most visible on-set presence from the vast ranks of a movie's various producers, the LP is the amanuensis of the funders, with the power to hire, fire and generally second-guess on a daily basis. Though not part of what is considered the 'creative' team, the line producer will liaise with the unit production manager, with whom a number of practical decisions about the logistics of the set may be shared.

Foley Artist
You thought:
- What the f*** is a Foley?

Actually:
The Foley artist's title is derived from the pioneering work of Jack Foley, who helped Hollywood make the movie to talkies by developing a technique of adding sound effects in post-production. Foley artists can usually be found in a dark room recording themselves as they tramp round a tray of sand, throttle some celery or saw up a pig's carcass.

Leadman
You thought:
- Ah hah, not the leading actor but a guy in charge of all those leads on film sets.
- A dense superhero.

Actually:
No. This guy (or gal) is in charge of the swing gang (see above) as well as the set dresser, whose job it is to arrange prop objects on the set and move it out of the crew's way as necessary. But he answers to the set decorator, whose job it is to choose the furnishing, environment and any in-frame objects not handled by actors (if they're handled by actors, the prop department is in charge).

AVID editor
You thought:
- A guy on a caffeine binge with a pair of scissors.

Actually:
A practitioner of non-linear editing (marking up digitised film with intros and outros for non-destructive re-ordering of the original footage). AVID was the flagship digital-editing solution of the late eighties and nineties, though its pro-level editor, Media Composer, has been challenged by Apple's Final Cut Pro in recent years.

Greensman
You thought:
- Immediately of Robin Hood and then realised that was just silly.

Actually:
Waters the plants and tends to any vegetation on set.

Music Arranger
You thought:
- The guy taking 10% of Danny Elfman's pay.
- The guy trying to make Danny Elfman's mad score work practically in the studio.

Actually:
Someone repurposing music for instruments and treatment that it wasn't originally intended for.

Off-line Editor
You thought:
- An editor who hates the internet.

Actually:
An editor who marks up preliminary cuts in a moderately-priced editing facility so that no undue time is wasted in the expensive suite of the on-line editor, who's next to get the footage.