How Do You RP (Outside what the developers intended)
Well it has always appeared the the developer (Bethesda, in the case of TES & FO3), has assumed that the player would consider themselves the PC (like an avatar in the game world), instead of considering the PC itself as the one that belongs in the game [world].
I prefer a view that includes the PC in the frame ~reminds me that they exist at all; and so I play TPP whenever practical (even though the game is not designed for this and has even lately been discouraging it... Like when NV crippled the Camera by locking it a few feet behind the PC's shoulder instead keeping it more in line with the series ~as a whole.)
As far as the other roleplaying aspects... there is only one [golden] rule as far as I'm concerned.
*And that is, "How would this particular person/or creature [the PC] react to the events taking place in the game; given their history and current goals". Stick with that and how can you go wrong?