Foget the theory! Use the common sense, kids. Get on stage, when nobody is around. What do you see? The house! The empty seats in front of you. The architecture itself is your "givens" -- like a landscape, river, ocean, forest, abis -- you decide which image you prefere, or have them all. Important that there is a difference between the space out there and your space on stage. It's obvious. No less that there is some line between the two.You not have to "creat" it, it exists, the so-called "fourth wall"... Imagine it? The idea is to imagine that there is no house! No public, nobody but you, that it is not theatre, and nobody wtches you! This is the real madness, because anybody on stage feels the presence of the audience, even when they are not there.
"Concentration"? Listen, when you daydreaming the whole world disappearing. Look, your character is fictional, it doesn't exist. As far is Hamlet is concern, there is no theatre, no public -- and no you. Paradox? Good. What could be better for drama and art!