overdubbing

The other day I was watching an episode of Reno 911 and I noticed a very obvious use of overdubbing. It happened in the episode when there was a prostitute character that was in the hospital that was being played by one of the other actors who is usually one of the main characters, Trudy in the show. The scene plays out with the other officers visiting this prostitute on her death bed and the actor that usually plays Trudy was acting as the prostitute, but Trudy isn't with the other officers, because she is playing the prostitute obviously. Her eyes are blurred out and she is wearing a wig, so it's not so obvious that it is her but being a fan of the show I could tell from the voice. Later the character Trudy comes and visits the prostitute, which poses a problem because she can't play both characters. This time the entire face of the prostitute is blurred out, because it is obviously another person playing the character. This is when the very obvious overdubbing comes in. You can tell the overdubbing because you can hear in the recording that it was done in a studio, where there is no reverb or echo and the voice of Trudy has the natural echo of the room. Another less obvious use of overdubbing in this episode comes when the actor of that plays Trudy is the prostitute and she "pukes" over the side of the bed and it splatters on the floor. The vomit is never shown on screen but viewers are convinced that it is happening because of the sound. The careful listener can hear that the moans of vomiting come from the character but the sound of splattering is added in afterwards. I guess what I've realized from all this is that overdubbing is very often used when situations are impossible or very difficult to achieve the real thing. As when a two characters can't be played by the same actor or when getting someone to actually puke would be very difficult.