Serifu (セリフ) is the Japanese word for dialogue (In anime industry, it's usually written in katakana). Animating to dialogue is usually called kuchipaku (口パク) "lip flaps." Along with the lip flaps, it's common for the jaw to move as well.
This is generally done with three frames. Frame 1: tojiguchi (トジ口), Closed mouth, Frame 2: nakaguchi (中口) MIddle Mouth, Frame 3: akiguchi (アキ口) Open Mouth. In the case that the scene want's closer lip syncing or varied mouths. A frame 4, and 5 will be added for the vowel shapes ウ and オ.
Creating Serifu in Layout[]
Single-Layer[]
This is the most common way of creating Serifu kuchipaku. The character is drawn in the first frame with a closed mouth. The second frame is left for the Douga artist. The third keyframe is drawn only as an open mouth. Optionally, Gousei (合成) can be used.
On the timesheet, the cels are labeled in a sequence of 3, enclosed with a parenthesis, and followed by the katakana text, セリフ. In the sound column, the hiragana syllables are written vertically to indicate the timing of the voice. This is written with an encircled shorthand of the characters name, with start and end points written as lines or single frame sized blocks.
Multi-Layer[]
In the case where the open mouth can fully cover the closed mouth, the mouth can be placed on a second layer, with the closed mouth remaining on the lower layer on the main cel. Generally this is less common in anime production and can make timesheets more complicated than necessary. There are exceptional cases where this method is used. In this case, the upper label will be labeled in a sequence of 3 with X being the first.
Douga[]
Douga artist will finish the mouth animation by drawing the nakaguchi (中口) . Generally the upper teeth will remain in place for this animation. In highly stylized shows, or characters that have big mouths may have the teeth move in place more.