Actors John Boyega (Finn) and Daisy Ridley (Rey) are fresh actors (just as Mark Hamill, Carrie Fisher, and Harrison Ford were in '77). Apparently, when they have not yet seen any post-editing floor footage of them selves and the movie, and only certain raw, isolated footage of their scenes, without any of the effects, they have not yet realized just how incredible (and history changing, cinema-wise) they are going to be when it has hit. During this state of ignorance, there is a point they do get to see the final cut trailer with the rest of us revealing a good taste of it... how would you feel in their shoes seeing yourself for the first time in the official world of Star Wars playing as an impactful lead? You would quite likely have the same reactions I bet.
https://www.youtube.com/watch?v=DXQBEBAoXjs