Cracking Animation says... (im typing this so there may be some things wrong)
Lip Sync
We match voices to lip movements by a somewhat ancient method which has been come traditional in the company. First, an editor marks all the syllables on the magnetic tape of the diaogue recording. Then he or she transfers all this to a bar chart, in which each bar covers a second of film (or 1ft of 35mm) so the animator can see exactly where each syllable starts and finishes. With the word 'Me', for example, the chart will show the 'M'-sound lasting for two frames, or whatever, and the longer 'eeeee' sound lasting for nine frames . If there is pre-recorded music, as there might be for a film which is set round a tune or song, this will also be sketched in, to show where the beats fall, or a particular run of notes which are important for the animator's timing.
The chart goes to the animator, who then copies the infromation on to a tradition animation dope sheet. There are 96 lines to a page, one line for each frame covering 4 seconds, and the lines run vertically down the sheet. On the dope shet you have more space to put other information, and that is the essential difference. Instead of having just a tiny gap per frame, as one the bar chart, here the animator has a whole line.
Now you have your own personal map of the sounds, and the gaps between them, and can begin to devise a performance for your character. From listening to the sound, you know when the voice goes up higher - which might indicate a questioning expression, with raised eyebrows.