Board Thread:Questions and Answers/@comment-2.98.50.212-20160427082522/@comment-91.157.76.247-20160427122410

Judging by what the Servants themselves say, they're not conscious when in the Throne, or at least they have no memory of their time there after the summoning.

I don't think the Servant's consent is required for the summoning ritual, as a prerequisite for the summoning is, in most cases, a wish for the Grail. Thus, they'd have no reason to refuse even if they are "conscious" at the moment of summoning.

Hope that answered your questions.