There hadn't been any wall there to feel, when she'd searched for it with her hands. Just more of the same dark air. There was a faint feeling of resistance around where it met the opening, though, like surface tension on water.

The Doctor's head, as he pushed it through, wasn't visible to Cayce. Neither of them, suddenly, were visible to each other; to Cayce's hearing the Doctor's voice was cut off as he entered. They could still see themselves, and the parts of each other that were touching on the other side (which, considering the size of the hole, were quite squeezed in together) but it all seemed to cut off at the opening.

The Doctor's link to the Master was still fully functioning, however.