How is sound usually added to cartoons during the production of them?
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Sound effects are added in post production, in a process known as Foley work. It is the same in live action film and animation. https://youtu.be/j6bVusny-Xk https://youtu.be/0IPxIvbc_cs
It usually depends on which format the animation is going to be produced: Movies or TV series. For movie formats, the recordings usually takes place before the animation was made. So the voice actors, with the help of the director, need to use their imagination and try to visualize about the scenes. For TV series, they usually use images from the scriptboard to help visualizing the scenes, and also with the help from the producer. There's also some cases that some changes to the scenes or needs a retake/re-record in late production, whether for movie or TV series. During this case, they usually used finished animation footage to help the voice actors improvising their lines.