Typically for a show with a decent-sized orchestra, the conductor would play the least prominent piano/keyboard part (ie - in the "Young Frankenstein" Broadway pit, the conductor also played Keyboard 3). Key 3 is used fairly infrequently throughout the show, so for the most part the conductor was conducting and not playing.
It can also be for "filler" - I saw an equity production of Camelot about 15 yrs ago with a 30 piece orchestra but the conductor played a synth with harp for only 1 number because it was the only number that "required" a harp (solo) so they did not feel the need to hire a real harp player.
In community/educational/regional theatre it is often because they want to get the most out of their budget so they conduct and play -- they may only have enough of a budget for 5 or 6 players in addition to themself.
Also in opera, sometimes the conductor will play the harpsichord recitative ---- but that is opera, not theatre !!
Updated On: 11/15/11 at 07:13 PM