My friend and I are having a debate about this.
The War Caster feat (PHB, p. 170) says:
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.
I believe this is to eliminate AoE spells. Can I use my reaction to cast Bigby's Hand with the War Caster feat?
My friend's argument is that the spell itself summons the hand, which you can then use one of its effects, and these effects don't necessarily target a creature (example, interposing hand just provides half cover).
My argument is that, since as part of the casting you can choose an effect and certain effects only target one creature, that you'd be allowed to use one of those effects only.
Also, if it's already cast, using one of Bigby's Hand's effects is only a bonus action, whereas the War Caster feat says that you can cast a spell with the casting time of one action. Technically, the spell does have the casting time of one action, but it's already been cast, and the attack would only be a bonus action. Would one of the effects of Bigby's Hand be allowed on the War Caster's opportunity attack if it were already cast?