The employee pays the union, but it's often the film production payroll offices that issues the actual check. Think of it like paying social security tax. Instead of having employees do the math and write checks to the government, you deduct the correct amount from their paychecks and do it for them.
Most actor contracts are between two and three pages long. The typical term for an actor contract is a certain a number of months after the film or play premieres, although some go on indefinitely. Term lengths can vary greatly! The typical term for an agent or manager contract for actors is about 12 to 18 months.
In film, everything the viewer sees needs to be arranged inside the frame. This creates meaningful relationships between the subjects. The points is to achieve a balanced, aesthetically pleasing, unified, harmonious whole. Don't put your subject in the middle.