We should be sure that we both are understanding Full Screen the same. Traditionally, Full Screen would chop off the sides of a 16:9 movie and just show the center most image to fill the tv screen. This was first done to satisfy folks who had a 4:3 aspect tv but I doubt that these are even made anymore.
These folks would rather watch a movie filling their screen than see the original aspect ratio as it played in the theater and as how the director intended it to be.
On the tablet, Full Screen means to fill as much as the screen as possible using the original aspect ratio. It does not chop off the sides of the movie to fill the screen.
Netflix takes advantage of the entire N10 screen but will have small horizontal black bars on top and bottom if the movie aspect ratio is greater than 16:9, the Cinemascope type movie.
With Amazon on Demand, Full Screen removes all the controls, ads et al from the actual movie screen but maintains the original aspect ratio. The initial view when the movie starts is Letterboxed but once you press the Full Screen button, it switches to the original aspect ratio.
I've only watched one movie on it so far, Rango, and it is a widescreen movie showing in what you called Letterbox. It's actually not Letterboxed as the screen touches both sides of the screen once you activate the Full Screen button. True Letterbox would be in the 16:9 ratio but with a black border on all 4 sides. The Amazon presentation is the original aspect ratio.
Most of the newer movies are in an aspect ratio greater than 16:9 so these movies will all have small black bars top and bottom but they will extend from side to side. Only a true 16:9 will completely fill the N10 screen. If you watch The Story of India in Amazon, it is 16:9 and pressing the Full Screen button wil give you the movie completely filling the N10 screen on all 4 sides.
In order to have uninterrupted viewing, however, you need to disable the screen saver which can't be done as far as I know. So setting it to the maximum time, 30 minutes, and then tapping the screen regularly should get you through the movie uninterrupted.
Netflix, thankfully, with its app just shows the entire movie as a default.
Shelly