This is something I was asked to do last week end, which is outside of my comfort zone. My perception - right or wrong - is that indoors, photographers usually shoot with zooms, and bounce flash with a bit of fill, creating a reasonably soft light with a bit of fill to get sharp images with neutral colour and no movement.
I hate flash.
I want to shoot with ambient light only, to get the atmosphere and colour of the event. Flash though, is almost essential to stop blurred faces and hollow eyes. My approach was to shoot with my Nikon D800 with fast primes, and just enough flash to fill shadows, but not enough to influence ambient light. ISO1600 and Almost everything shot at f1.4. The results are a bit more unpredictable - per shot, but there's more colour and ambience.