For starters is will do actually =)
Of course, using mirror is the easy way. I would definitely never show anyone a face I am making in front of mirror when I try do do the face for expressions like these, for example. It's one fugly mug
But then it's when tutorials, emoticons and other people photos come up. You have 2 main characters, save yourself a nice photo library of different expressions and try to sketch your characters with those faces from different angles! Find 6 emotions your characters should express, for example. Again, just take your time with sketches! I did this thing first time as a part of character challenge in 2016 (so it doesn't look great now, heh) and it helped a lot, really.