The Gaffer will have a Best Boy and a team of electricians, Sparks, who will work on the lighting plot with the Director of Photography (aka Cinematographer, Lighting Camera). Although much of the work will take place when the actors are off set, using stand-ins, there will be plenty of final touches during rehearsals. The lighting team will bring to life the writer and director's vision for the look of the film. This means lighting for day or night, interior or exterior, summer or winter, regardless of where and when you are actually filming, (Michael Powell's 'Black Narcissus' set in stormy Himalayan mountains was shot entirely in Pinewood studios and a garden in West Sussex).
The job of a Grip in the UK involves the equipment that the camera is mounted on. If the camera has to travel at all, it will usually be on a Dolly that moves along tracks. Hence the term Dolly Grip. The Dolly can also be used for moving the camera up and down. The Key Grip is the chief, and sometimes only, Grip. The Grip has to move the camera precisely, often in difficult conditions. Complicated camera moves, in tight spaces, with particular timing that is dependent on performance, mean that the Grip, along with the Focus Puller and the Camera Operator, can be under a lot of pressure during a take.
You can make the life of the Gaffer and Grip easier; you can hit your mark, you can go through a rehearsal at the right speed and with a decent summary of the action. It's always worth remembering: they're in charge of the lights and the camera.