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.

Untapping an attacking creature doesn't remove it from combat.

Act of Heroism can target an untapped creature. It still gets +2/+2 and can block an additional creature.

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.