When we first played we missed that part of the rules and actually went further than that and let the Reanimate take its turn at any point during the hero's turn, completely separated from the Necromancer's turn. Chances are we'll probably keep playing that way anyway since it doesn't really make any difference.
I think the main reason for doing it before or after the Necromancer's turn is so that you can still easily keep track of whether it has been activated yet this turn, since it doesn't have its own hero card to turn over. I don't think it'd actually be a problem, but if it was you could always just "exhaust" the Reanimate card as a reminder.
Please just a little more indulgence. Let me refocus away from Standing-Up to just spending your two actions normally. Many people, not including me, are saying a hero cannot spend fatigue straight after his second action because his turn ends immediately.
Can the Necromancer spend fatigue immediately after his second action?
Can any hero spend fatigue immediately after his second action?
@Terah: Even if you couldn't it would not matter. You can spend the fatigue for points while you are moving. You don't have to wait till after.
Sapiens qui vigilat
That's definitely not true (according to my understanding of the rules.)
Flipping over your activation card to end your turn is a completely separate step of the Hero Turn sequence than taking your actions for the turn, so there's definitely room in between taking your second action and flipping the card for you to do other things, like spend fatigue or activate a familiar.
In the specific case of the Stand Up action, flipping over your card is stated as part and parcel of resolving the action, so in that specific case you can't do anything else because ending your turn is literally part of the action. However, to the best of my knowledge, no other action calls for the hero to flip his activation card within the action's resolution text.
MP3 killed the radio star
Absolutely correct. Flipping your Activation Card is an actual step in the process separate from taking actions, but Stand-Up effectively forces you into the final phase of the player's turn.