Voices in One Room

A method of recording voices where all the actors are in the same room and essentially record their dialogue together. This is the default way of recording dialogue for Anime; it allows for very quick production schedules, since most anime is actually drawn before it's voiced. This also allows actors to specially play off each other's performances as well as see how the actual character looks while the dialogue is spoken.

Traditionally, Western cartoons complete all their voice work before the animation is done, and the animator's job is to match the actor's performance. This can lead to higher animation quality. However, due to the nature of the industry many performers are only voice actors part-time, and usually cannot be gathered together for a single group recording session because of scheduling conflicts. This is sometimes seen as the cause of wooden dialogue.

Examples of Voices in One Room include:

Anime

The staff of Code Geass made a comic detailing the process, discussing the actors' (represented by their respective characters) particular quirks, like Takahiro Sakurai wearing a beret to get "psyched up" for his reads and the male actors teasing Ami Koshimizu about how thin her thighs are.
This was averted in the English dub of Sailor Moon. Incidentally, Sarah LeFluer and Barbera Radeki (who played "Michelle" and "Amara", partners in a bowdlerized relationship), always recorded together.

Video Games

The English version of Snatcher had actors that recorded with each other. This combined with the use of actual professional voice actors made for a result way better than what most games had at the time.
While most of the voice actors in the English dub of Kid Icarus Uprising recorded their lines separately, Pit and Palutena's voice actors, Antony Del Rio and Ali Hillis respectively, recorded many of their lines together.

Web Original

Sailor Moon Abridged is filmed in one room, shown in the outtakes when things get changed in the last minute, lots of Throw It In and one of the actors being forced to leave the room because he couldn't stop laughing and ruining the take.

Western Animation

When they did The Road to El Dorado, Kenneth Branagh and Kevin Kline asked to perform their dialogue together. The result was a very natural (and very funny) banter.
This is the standard procedure for any show in the DCAU, and appears to be voice director Andrea Romano's preferred style.
- At times Mark Hamill would just accidentally mess up the session by making everyone laugh while doing the Joker voice. How? Because he didn't sit down and do it, he stood and do actions and stuff. At times, the artists would then base Joker's movement during lines directly off of what Hamil was doing. The reason this worked is because this is the guy who played Luke Skywalker, who could overact like no other (NOOOOOOOOOOOOOOOOOOO!!!! IT'S NOT POSSIBLE!!!!!!!!!!), which fit the Joker.
  - In one famous example, the episode The Man Who Killed Batman has Joker conduct a "funeral" for Bats, complete with Harley playing "Amazing Freaking Grace"... on the kazoo. They could only do this in one take because Arleen Sorkin actually did play "Amazing Grace" on kazoo and afterwards everyone was laughing too hard to continue.
- In an interview for Batman: Arkham Asylum, Kevin Conroy (the voice of Batman) mentions that this is how most of the WB's animated shows are produced. By comparison, trying to give a comparable performance alone in a recording booth is much harder.
For the Sonic the Hedgehog cartoon series, a variation was used: Most of the cast (which was largely composed of veteran full-time voice actors) acted in the same room, with the sole exception of Jaleel White, the voice of Sonic. While this made it hard to play off of the rest of the cast, he also had more takes to work with.
Done in production of Foster's Home for Imaginary Friends.
Notorious on Angry Beavers, leading to a general conversational feel, as well as tons and tons of very obvious Throw It In.
This trope and Improv are how the CGI animated film Surf's Up was recorded.
John Goodman and Billy Crystal recorded most of their dialogue together on Monsters, Inc..
Thundercats had the cast recording in the same room, leading to some amusing bloopers (with many expletives)
The original Transformers cartoon was recorded like this, and according to the voice actors this led to many amusing antics inside the recording studio—Frank Welker and Michael Bell were apparently notorious for encouraging the voice actors to misbehave.
Recess: School's Out had all the voice actors record together, while for the regular Recess series, only the main six kids had their voices recorded at the same time, as with the Ashleys.