<p>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.
</p><p>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.
</p><p>Hope that answered your questions.
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