“trembled.” The Hebrew is hard to exactly translate because it can have different meanings. It can mean “trembled” for some reason such as fear, but in this case Boaz might have simply been chilled by the night air. Or Boaz may have in some sense felt something was wrong and trembled and turned, only to discover a person was there.
“behold, a woman was lying at his feet.” The word “behold” in this context indicates surprise and probably even shock. The presence of Ruth there at the threshing floor and there at his feet was a total surprise. It likely took him a moment to even figure out that it was a woman. The Bible is silent about how much light there was at that time from the moon, but judging by the fact that Ruth could leave without being well seen there would not have been a full moon that night.