For TS, if spotlight is available and you would like to keep the mood by taking in the ambient light, I would suggest using high ISO. Here are two examples:
These pics were taken at DXO with a compact camera with ISO manually set to 1600 at f/2.8 and without flash. If you're shooting with a DSLR you can definitely go with higher ISO. It is unavoidable that you'll get some noise into the pic, but comparing a flat pic with the ambient light destroyed by flash to one that retains the ambient but with noise, I would say the later is a better choice. Furthermore you could apply noise reduction later during PP to cut them down.
If the place is so dark that you can't even auto focus and with the highest ISO and your shutter speed is still not up to acceptable level, or the ambient is ok but your subject is too dark, you can experiment with reduced powered fill flash by using either flash compensation or manual flash output power setting.
Good luck and have fun.