You may have noticed that we start and finish pieces without somebody waving their arms at the front. When preparing a concert, our biggest concern is not whether performers can count up to four (because we always check first). Most music in Baroque times was performed without a conductor. There was some direction, usually by the composer (with, of course, an overview of the music hed written) from an instrument.
"Interpretative" conductors, as we know them today, are a legacy of the 19th century Romantic cult of the ego. During the Romantic era, the artist was no longer a servant but a free man, free to express his suffering. The interpretative conductor was an integral part in the performance of great works celebrating such self-indulgence.
We cannot deny that in some pictures from the early 18th century,
there is an otherwise unoccupied man, usually holding two rolls of paper: one for dynamic, the other for tactus (although in this one, it could be a moody flute player snapping his instrument).
For large choirs, opera, big music (Handels Music for the Royal Fireworks, for instance) and complex polyphonic pieces with several groups scattered about a cathedral, it is helpful to have visual cues when some performers are too far away from the central continuo group to play in time with it. Barokensemble de Swaen do perform with conductors when this is the case.
Whereas nowadays many romantic conductors make faces, draw special runes in the air and neglect their basic duty of metrical indication, those who are clear, practically-minded and respect the other musicians can add something very special. Last December, for instance, we performed JS Bach's Christmas Oratorio with the 100-strong Bachkoor Nijmegen, brilliantly directed by Rob Vermeulen (who today, is singing in the choir and giving some cues). Without Rob at the helm, this could not have been such a thrilling experience for performers and audience alike.
On the subject of choirs, One Voice Per Part, or OVPP is another hotly-debated HIP topic. In the early 1980s, Joshua Rifkin had very good reasons to propose that cantatas in 18th century Leipzig were normally performed with one singer and one instrument on each of the parts. Since OVPP had been an unbroken tradition for at least 200 years beforehand, most people involved in HIP accept that it was probably still the case in the early 18th century. Of the few that object, many are German academics that also conduct big choirs and are therefore uncomfortable with this conflict of interest. We normally perform OVPP but today, because the music is so festive, youll hear an augmented choir with three singers per part. Hurrah!