The plot Fortified Position reads "Treat each character as if its printed text box were blank (except for Traits)".

Is it implied that this card only affects characters that are in play, and those which subsequently enter play while the plot is revealed?

Or does it also affect out of play cards (ie. preventing ambush of characters)?

It only affects cards in play. The Rules Reference explicitly states as much on p2. In this case, fortified position does not prevent ambush.

x x Card abilities only interact with, and can only

target, cards that are in play, unless the ability

text specifically refers to an out of play area or

element.

I had a follow up to this question. At what point does Fortified Position take to effect. For example, the 'Queen's Assassin' says: "Reaction : After The Queen’s Assassin enters play using ambush, choose an opponent..." Does this ever go into effect or is it immediately null'd by 'Fortified Position'

It takes effect as soon as a card enters play (or as soon as Fortified Position is revealed) - instantaneously. It's what is termed a "constant" effect. That is to say that by the time you get to the reaction window OF a character entering play, that character is already blank, and could not initiate that ability.

And another follow up to this question.

What is 'printed text box'? I asume everything including gold and reserve bonuses on the cards, not just the text?

Rules Reference, page 4, Blank (which is refered to by the entry on Printed)
"xxA card’s text box includes: traits, keywords, card