Basically, a best boy grip is the assistant to the key grip — the man or woman responsible for making sure cameras and lights are where they are supposed to be. Grips are technicians who work in film, interacting closely with two departments: the camera crew, and the lighting crew. Grips may be called on by the camera department to set up camera dollies, mounts, trailers, or cranes. In many parts of the world — including Australia and the United Kingdom — this is the extent of a grip’s purview. In the United States and other nations, however, grips also interact heavily with the lighting department and crew. Although union-job rules in the United States prohibit a grip from actually handling the lights, they work closely with the electricians who do that work. They also deal with light stands, nets, diffusion tents, bounces, flags, and large outdoor tents, all of which help focus, direct, diffuse, or cut off light.
The key grip runs this entire operation, essentially acting as foreman for all of the other grips on a set — which in the case of a large movie may be dozens. The best boy grip acts as the right-hand man to the key grip, handling logistics and orchestrating the other grips on a large set, or handling the bulk of the equipment on a small job. He or she has many jobs on a set, and these jobs range widely in scope. He or she will likely be responsible for hiring and firing personnel, coordinating the grip crew with the film or electrical crew, planning rigging and lighting schemes, and many other jobs. The best boy grip is also usually the contact person between the entire grip department and the unit production manager on site.
Best boy grips have a counterpart in the electrical department, usually known as best boy electric. Best boy electric serves much the same role, except serves under the gaffer, rather than the key grip.
The name grip is a holdover from old circus slang, and may have come from a term used to describe the small bag of tools all grips seem to carry with them. The phrase best boy grip is a holdover from the old guild craft system, where a master would have many apprentices, and the oldest — and therefore the one with the most responsibilities — would be called the best boy. Despite the inherent gender of the term, a best boy grip may be either male or female, and women make up a sizable minority of grips.