Inside and out
Because we all know that this is the sort of thing that turns our mad doctor on. The title is taken from the 31 Days theme for October 23, 2007. Oh yeah: note that this piece contains spoilers for the Medusa Arc of the anime and manga.
Stein enjoys himself the most when they struggle, thrashing against the leather buckles strapping them down to his metal table, one that he doesn't bother washing clean because there's no real need for sanitation during an experiment like this. He likes it when they beg or curse – it's the only reason why he doesn't shove something in between their teeth to keep them from making too much noise. The blindfold, though, is necessary: too many of his specimens have died too quickly out of the shock that comes with watching someone cut their body open. It was in his best interests and in theirs that they did not exactly know what was coming.
He times himself during the procedures, checking to see if he can break his last record for enduring in as much as he can before the sight of a sweaty, warm body being systematically reduced to a quivering mound of neatly dissected flesh drives him mad. His reward for surpassing his limits is the pleasure of finger fucking his specimen if it is female, and jerking it off if it is male – it is most interesting, watching his specimen's faces as they find themselves torn between excruciating pain, warm pleasure and certain death.
He would have wanted to do this to Medusa, had she survived their fight. It would have been nice, tying her down and sewing her back together only to cut her apart again.