How did they do that?
Montgomery played both Samantha and Serena on Bewitched.
Elizabeth Montgomery played both suburban housewife Samantha and kooky cousin Serena only in the sense that all of the spoken lines were performed by her. Visually, there were tricks, and usually two people were involved.
There were four basic techniques used for both Samantha and Serena to appear in the same scene:
1) The simplest method was to show only one character at a time. EM would appear as Samantha in a, generally, close-up shot of Samantha only and read those lines. Then, the camera would switch to a shot of EM as Serena only, and she would read Serena’s lines. This would be repeated until the scene was done. In reality, EM would read all of Samantha’s lines for the entire scene, change into the Serena costume and read all of Serena’s lines. The film would then be cut and spliced, so it appeared that the two were conversing.
2) The next level of visual trickery is what Scott Viets (The Bewitched Critic) refers to as “back acting.” In this case, you always see the face of either Samantha or Serena and the back of the other character. Typically, EM would read all of Samantha’s lines with the camera looking over the shoulder of someone (e.g., EM’s stand-in Melody McCord) wearing Serena’s black wig. Usually, this technique was used on a close-up shot; however, on occasion there would be a full-length view of Samantha and Serena. This was always a back or three-quarter back view, so that the face of only one character showed at a time. Then, EM and the stand-in would switch costumes and roles, and EM would read all of Serena’s lines. Again the film would be cut and spliced, so that the scene seemed to progress as a normal conversation between the two characters. On rare occasions, and always on distant shots, EM’s voice would be dubbed in for the voice of the character whose back was showing. This was first seen in # 54 and other good examples of this are in episodes 161, 189, 205, and 231.
3) The most difficult approach, technically, is the use of the “blue background” system. In this mode, both Samantha and Serena were seen at the same time with full-face views of both. Here EM would read Samantha’s lines with someone else reading Serena’s lines so the timings of Samantha’s speech would be correct. Then, EM would be filmed as Serena against a plain (usually blue) background with Samantha’s lines being read to keep the timing accurate. At this point, the two sets of film would be merged with the extra voices dubbed out. This system worked fine, but was the most expensive and time consuming to do, plus both characters had to be relatively still.
4) If frontal views of both characters were required, and there was considerable movement, then two people were used. EM would take the role of the principal character,(i.e., the one with the most lines to say), and the stand-in actress (always Melody in this mode) would take the other part. In this case, there would usually be an attempt to keep Melody’s face from showing. Hiding behind hair was a favorite ploy, as was moving behind other actors so that her face was not fully exposed. This technique was not used often, and is most noticeable in the following episodes: