D&D 5E (2014) Magic Circle and not-conjured creatures

[arunwe2012]

Hi all, I want to know your opinion about this: do you think it's possible to cast a Magic Circle on a creature already present on the battlefield, instead of one conjured?

 

[Setryo]

If said creature is incapacitated, I'd say it's possible. But if not, the casting time of Magic Circle is 1 minute. You can't reasonably expect for an elemental/angel/whatever to just stand there politely while you draw a circle in magic chalks around it and chant magical words.

 

[arunwe2012]

That's what I think, but the wording of the spell doesn't say if you select the point of effect before starting to cast or after casting. If it was the second, then you should be able to trap the aforementioned creature.