我在国外网站看到这么一个解释,总结起来就是:
1、sam和white walker对视镜头,其实都是独立的画面,然观众觉得他们互相看见了而已,实际上sam并没有真的被看到;
2、异鬼前进的时候,雪地里有几块不太大的石头,而sam趴在雪地里的衣服、斗篷看起来实际上是和这些石头差不多的,所以他很可能是趴在石头旁边保持静止,假装石头瞒混过去了。
Now, it may very well have been just your good old fashioned MacGuffin, HOWEVER on closer inspection of the scene at the end.
You never actually see Sam or the mounted White Walker make eye contact. It is cleverly implied. Sam clearly hears the hooves padding through the snow and hears the rattling chains and the White Walker looks off to the left and then shrieks an order/battlecry.
BUUUUUUUUUUUUT, when the pan out begins, we see: The large rock is IN FRONT of the mounted white walker.

The only other rock in the entire pan out is tiny and there is no shallow on the other side.

And Sam is clearly seen getting behind a rock with a shallow which, when he sits down, gives about a foot of cover above

Therefore
The walkers never actually saw him
He pressed his back against the rock and waited until he thought they had all gone past
He then legged it!