“he was standing in the selfsame spot you're filling now”, the sentence refers to real place or role, or position?

The sentence is rather poor English but, by using the word "spot" (meaning a physical location on the ground), it clearly means "He was standing in exactly the same place as you are now standing." Since a spot is a physical object, either real or imaginary, it would be more correct to say "on [the spot]" rather than "in". The preposition "in" is used for "place", "position", "location" and so on; a 'spot' (unless it is used in an idiom like 'beauty spot', where it means a location or an area) is a small, physical two-dimensional object, either real (as a mark painted on the ground or, for example, on a billiard table) or imaginary, on a surface and, since it is assumed to be small, one cannot usually think of something as being 'in' a spot. Although "selfsame" is a word with a venerable history, it is now considered rather antiquated or 'poetic'. Modern equivalents are 'exact', 'identical', 'exactly the same'. The juxtaposition of the ideas of "standing" and "filling" is odd (and unnecessary). The whole sentence shows that the speaker probably had two alternative ideas in mind and has mixed them up when uttering the statement.

He was standing. = person in action, verb is standing, standing=doing You're filling now = you are filling, verb is filling, filling=doing in the selfsame spot = spot is a place, selfsame is an adjective describing the place, selfsame=same as your own self, as used in the sentence identical to another he/you It means another person has done what you are presently doing now. To say standing in means to carry on in the same way.