What is it about seemingly arbitrary people that makes them attractive?

Sometimes when I have begun a movie, and been presented with various actors, I don’t think they are that attractive. Then as the movie progresses they seem to become much more attractive. How is that? But mostly what I find so interesting is ordinary people who are thought of as very attractive, like Helen Mirren for example.To me she seems quite ordinary really, but she is a sex symbol. Which is great I think, since it shows us that there are different types of sexiness.

However, how does one explain this? What is the phenomena of it? Is it the “X” Factor? If so do you experience X Factors around you? and what is it exactly if you had to describe it?