燈影兒後面 Behind the Lanterns Shadow
說不了，就有一個漢子來牽馬，行者把馬兒遞與牽進去。他引著師父，從燈影兒後面，徑上樓門，那樓上有方便的桌椅。说不了，就有一个汉子来牵马，行者把马儿递与牵进去。他引着师父，从灯影儿后面，径上楼门，那楼上有方便的桌椅。
逐句解 Line by line
Before she had finished speaking a fellow came out to take the horse, and Pilgrim handed it over to be led inside.
了 is liǎo here, to finish — 說不了 is before the words were out, not the light particle the same character usually is. The horse goes first, which is what happens at a real inn and also what a horse trader would expect.
He led his master up behind the shadow of the lantern, straight through the upstairs door, where there were serviceable tables and chairs.
The detail is a smuggler's. 燈影兒後面 is behind where the lantern throws its shadow, and Pilgrim is routing Tripitaka through the one strip of the room where a face cannot be read. 著 is zhe, the leading held in progress.
Four men in stolen clothes with a shaved-headed master, walking into a lit inn. The lantern's shadow is the whole margin of safety and Pilgrim has already measured it.
Notice how little of this is magic. He could turn Tripitaka into anything he likes. Instead he walks him up the dark side of a staircase, because a transformation would have to be held all night in front of strangers and a shadow does not.