Everyone is familiar with the many voices of Mel Blanc (seen above with Director Chuck Jones (left) and fellow voice actress June Foray), who gave life to most of the Warner Brothers cartoon characters, and you may be familiar with other voice actors such as Dan Castellaneta and those found on The Simpsons. Many prolific voice actors such as Don Messick and Bill Thompson lived and died in obscurity, but their ranks are swelling with the infusion of A-list actors such as Cameron Diaz, whom producers seem to think are worth paying $5 million for two weeks' work, and many B- and C-list actors such as Mark Hamill, who now cannot get any type of acting work other than voice.
Typically, voice actors record their lines without other voice actors present, as they are often coached one-on-one by the director as to the specific pronunciation, nuance, emotion, and emphasis desired. But, if the voice actors have worked together before and know each others' characters well, they often record together, playing off of each other to maximum effect. A good example of this is the combination of John C. Reilly and Sarah Silverman in Wreck-it Ralph 2.
There's a lot more to voice acting than meets the eye, and, if a voice actor is established, he or she often is allowed to contribute to the scripting ("my character wouldn't say that") or even ad-lib, as Robin Williams often did in the Disney animated film Aladdin. Voice actors can have amazing longevity compared to screen actors; Mae Questal voiced Betty Boop from 1930 to 1989, and Mel Blanc voiced Bugs Bunny and others during the same extended time frame.
Following are some select voice acting videos found on YouTube, to give one a better flavor of what voice acting is all about. These are just samples; dozens are available, so why not look for your own favorites?
Mel Blanc voicing live radio in 1944.
Voicing Wreck-it Ralph.
Recording in the Toy Story franchise.