Reading In

Sometimes you have a character answering a telephone call, with another character speaking to them on the other end. In order for the first actor to be able to respond effectively, someone will usually stand off behind the camera, "reading in" the lines.

Most of the time this is a member of the crew, but there are exceptions, where the second actor does it.

Exceptions

 * 24, a major Split Screen user, almost always has the other person present.
 * Doctor Who, "Silence in the Library" David Tennant was present for the filming of a scene involving The Girl and read in his lines.
 * Averted in "Blink", where the actors actually played off a pre-recorded video clip.
 * Firefly, "Objects in Space", Summer Glau read her lines as "Serenity" just offstage.
 * This was averted in Season 1 of Heroes when Sendhil Ramamurthy had to play Mohinder making many, many phone calls from his apartment, but the audience never hears the other side of the conversation. This happened so often that when Mohinder met with Nathan Petrelli in an episode, someone remarked on the commentary, "Hey! Sendhil's in a scene with another human being!"
 * Done in UHF. George (Weird Al Yankovic) leaves a hilariously pleading apology on his girlfriend's answering machine. This was accomplished by having Al stand just out of shot and yell his lines while the girl reacted.
 * An odd version of this happened in all Star Wars movies involving Darth Vader. David Prowse (Vader for IV V and VI) and Hayden Christiansen (ep. III), both actually read their lines while in the suit, waiting for James Earl Jones to dub in the lines later on (though Prowse, in the first film, was under the misapprehension that he was going to be actually speaking the lines in the movie). This helped keep the twist in Empire that Vader was Luke's father by having Prowse declare on stage that "Obi-Wan killed your father." It was left to Jones to dub the correct line in later.