Can't speak to the Royal Marines, but as an infantryman I can't stress enough that it is not simply a job change but if you are to be successful its a complete lifestyle change. Military life forms you to its mold. If you can't learn to live within the mold your time of service will not be enjoyable. All that aside even a Royal Marine could talk till he was blue in the face, w/o going through the experience you won't really understand what it entails.

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"People sleep peacibly in their beds at night because rough men stand ready in the night to visit violence on those who would do them harm. "

George Orwell