Act of Heroism can target an untapped creature. It still gets +2/+2 and can block an additional creature.
16-03-2018
wotc
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.
16-03-2018
wotc
Untapping an attacking creature doesn't remove it from combat.
16-03-2018
wotc
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.