I’ve seen it portrayed both ways in historical fiction. Outside would seem more practical, given the enclosed space of a classical temple’s cella or inner sanctuary; you wouldn’t want it filling up with smoke from burning animal fat. But inside is, well, where the god lives.
As far as I know, the sacrificial altar would be outside the cella, for obvious reasons, and there would usually be a smaller altar inside the cella for incense, libations, and other things that don’t require killing an animal, in front of the statue of the god. Cellae were pretty small.