Usually, "most" means "the greatest amount" or "the highest" of some feature/attribute. And "more" is used the same way to compare things. Of all my friends, she has the most beautiful smile. She has a more beautiful smile than her sister does. But "most" can also mean "very, extremely, quite, rather". So you can replace "most interesting" with "extremely interesting". That is the meaning of the following sentence: The strange thing was that at first sight he looked most interesting. The strange thing was that at first sight he looked extremely interesting.

"Most" is the superlative form. Use this to compare more than two things. In this case, however, "most" is a synonym, or has similar meaning, to words like best, highest, and greatest. The meaning is that he looked very interesting. "More" is the comparative form. Use this to compare two things. (He looked more interesting than she did.) I hope this clarifies...please ask if you need further explanation!

In my opinion "most" in this context means very and is not used in the sense of comparison. "more" would not fit. "more" is often used to compare two things. "most" is most often used as a superlative.

Here, "most" is not being used to make a comparison. "Most" is sometimes used to mean pretty much the same as "quite," and that's what's happening here. So, it's saying that at first sight he looked quite interesting. There is no equivalent expression that uses "more." "More" is only used for comparisons.