Just learning Lip sync is using the dialogue that you have been given a disservice, as there is so many great ways to interpret and notice subtext in a bit of dialogue.
The way that you as animator approach a dialogue scene can make or break your career as the strength of your animation skills become obvious in a shot with only the thought of your acting skills (it's weird thinking that animators need to be also actors but there is a book about it that you have to read "acting for animators" by Ed Hooks) come into play and how observant you are to the world around you (There was a lot that came up when I looked online for "inflection" not surprising more so then this post from the Bernad L. Shwartz communication institute on the way that we speak).
A few good posts on the subject from other blogs.
About the lip sync mouth shapes shouldn't be trusted from Idle worm Or Subtext 101, the Clarification of Subtext on Brendan's Body Blog and tips to Animate Dialogue through Animation tips and tricks.
Few notes from the lecture with Matthew Munn (From: http://www.manar-tawam.com/2013/01/am-class-5-week10-lecture.html):
- Subtext is very important in animation because its a powerful tool to add reality to the shot.
- Subtext can go a long with the spoken words, or it could be completely the opposite.
- As an animator you have the time to inject real life performance into the characters, & add a subtle movement to make the characters look believable.
- Subtext will reveal the nature of your character to the audience.
- Subtext is not an afterthought, its something to be considered in an early stage of animation.
- Ask questions about your character, What kind of person is he? whats the mood he's in? what he's thinking?
- Before shooting a video reference, put yourself in your character's shoes, think the way there supposed to be thinking when delivering there line of dialog, and subtext will show naturally in your reference.
- In the polishing stage, pay extra attention on emphasizing the subtext.