Frodo in the Tower of Cirith Ungol

[Huinesoron]

It's also worth considering Frodo's state of mind (and body) at this point. To take a few select quotes:

He was naked, lying as if in a swoon on a heap of filthy rags: his arm was flung up, shielding his head, and across his side there ran an ugly whip-weal.

`Frodo! Mr. Frodo, my dear!’ cried Sam, tears almost blinding him. `It’s Sam, I’ve come!’ He half lifted his master and hugged him to his breast. Frodo opened his eyes.

`Am I still dreaming?’ he muttered. `But the other dreams were horrible.’

`You’re not dreaming at all, Master,’ said Sam. `It’s real. It’s me. I’ve come.’

`I can hardly believe it,’ said Frodo, clutching him. `There was an orc with a whip, and then it turns into Sam! Then I wasn’t dreaming after all when I heard that singing down below, and I tried to answer? Was it you?'

[...]

`Only that?’ said Frodo. `It seems weeks. You must tell me all about it, if we get a chance. Something hit me, didn’t it? And I fell into darkness and foul dreams, and woke and found that waking was worse. Orcs were all round me. I think they had just been pouring some horrible burning drink down my throat. My head grew clear, but I was aching and weary. They stripped me of everything; and then two great brutes came and questioned me, questioned me until I thought I should go mad, standing over me, gloating, fingering their knives. I’ll never forget their claws and eyes.’

[...]

`They’ve taken everything, Sam,’ said Frodo. `Everything I had. Do you understand? Everything!’ He cowered on the floor again with bowed head, as his own words brought home to him the fullness of the disaster, and despair overwhelmed him. ‘The quest has failed Sam. Even if we get out of here, we can’t escape. Only Elves can escape. Away, away out of Middle-earth, far away over the Sea. If even that is wide enough to keep the Shadow out.’

Frodo was still suffering from the effects of Shelob's poison when he was 'questioned' (whatever form that took). He was delirious - look at the nightmarish description of the Orcs, with his focus on 'their claws and eyes'. He was still not entirely connected with reality when Sam showed up - 'There was an orc with a whip, and then it turns into Sam'. Not 'and then Sam rescued me', but 'it turned into Sam'.

He was also filled with Orc-draught, which could have interacted in any number of ways with spider-venom. What does happen if you mix a powerful sedative with a powerful stimulant, both of them steeped in the Shadow? I'm gonna go out on a limb and say 'nothing good'.

And finally, he was (once he realised what was happening) in the depths of absolute despair - so far as he knew, the Orcs had the Ring. Look at that description of his failure; it has the lyrical construction of utter misery.

So: delusional, drugged both up and down, and sunk into despair. He thinks the Orcs know exactly what's going on - their gloating over capturing him reads, to Frodo, as gloating over reclaiming the Ring. Their questions, then - who are you? Why are you here? Is there anyone else with you? - must feel like purest mockery. They can't be expecting him to answer (Frodo thinks), because they already know everything that matters.