Three Lifting Questions: Rest Time, Secondary Muscle Groups, and Shoulders

There are different theories about how often to exercise muscles. I have found that exercising the same muscles twice per week provides ample rest time. Then you hear stories about actors in movies who work out four hours per day every day in order to bulk up for a role (Leonidas in 300). My first question is, assuming that the emphasis is more on muscle building and less on cardio, what is the appropriate amount of rest time for muscles? If they are worked out more often do they adapt? Think about members of the military who are in boot camp and exercise most of the day.

Then there is the question about the relationships between what I will call primary and secondary muscles. Primary muscles are pecs, lats, quads and hams. Secondary muscles are bis, tris, shoulders, abs, lower back, forearms, calves, and traps. Clearly triceps play a major role in chest exercises. A percentage of a bench press is performed by the pecs and another percentage by the tris with a huge overlap in the middle of the rep. The stronger the triceps, the more weight you will be able to lift even though the focus of the exercise is on pecs.

Let us assume that you have been lifting for a couple of months and have acclimated your muscles to a strong, stable state. Will your muscles grow bigger, faster, and stronger if you work out the primary muscle and its supported muscles in the same day or on separate days? As a follow up to that last question, let us assume you are doing primary and secondary muscles on separate days. To what extent does having a "sore" (meaning noticeably worked) secondary muscle - like your biceps - cause the primary muscle it supports - like your back - to take on the the bulk of the load and therefore get a better exercise for that primary muscle?

Lastly, when the hell do you do shoulders?