Act of Heroism

• WWizards of the Coast2018-03-16
  Official Ruling
  Act of Heroism can target an untapped creature. It still gets +2/+2 and can block an additional creature.
• WWizards of the Coast2018-03-16
  Official Ruling
  You can cast Act of Heroism even if the target creature won't be able to block right away, perhaps because you're the attacking player.
• WWizards of the Coast2018-03-16
  Official Ruling
  Untapping an attacking creature doesn't remove it from combat.
• WWizards of the Coast2018-03-16
  Official Ruling
  The effects of Act of Heroism are cumulative. If multiples resolve targeting the same creature, that creature can block that many additional creatures this turn.

Rulings via Scryfall