I reckon its all about the bottle and the flow of penelties (ie whats happened previous to the one you're taking).
I think I read something after Celtic beat Spartak on penelties to enter this seasons Champions league about how the penelty takers were selected. Rather than say 'who fancies it then?' or 'sort it out among yourselves, it's a lottery' , Strachan picked the kickers himself and told them they were taking a penelty.
It's an interesting alternative approach to take than the scenarios mentioned above and I think would give the takers themselves a bit of confidence. Would Irelands takers against Spain have been different/ performed any differently???