How do you roast an entire eggplant in the oven? And, when is it important to score the eggplant with cross-hatches?

I cut mine in half before I roast it and lay it skin side down in tin foil. That way I don't have to contend with eggplant juice dripping all over the oven.

Roast until it's nice and browned on top and soft (you can score it if you like) and then scoop it out of the skin.

Is it important to pierce the eggplant if you're going to char it over the flame of a gas stove?

I love to roast eggplant in my wood burning oven by burying it in the coals. It is creamy and smokey flavored.

You can do the same thing on a grill, too. Just keep the eggplants over indirect heat until the skin looks like it has deflated and the inside is very soft.
Scoring the eggplant with crosshatches is a good method if you want to roast your eggplants split in half. Score the flesh and pour olive oil over it. This results in a rich, creamy eggplant. I've also done this and spread a miso paste over the flesh. The crosshatches allow the seasoning to permeate the flesh.

Agree with Monita's answer, and she's not joking - I have put a whole eggplant into the oven without pricking the skin, and it does indeed explode and leave strangs of eggplant all over the inside of the oven.

One tip for a smokey flavour - prick the skin of the eggplant, toast it on top of a gas flame until starting to char, and then pop into the oven.

To roast a whole eggplant just prick it with a fork the way you would a potato so it doesn't explode. Roast at 400 until it deflates and the flesh inside can easily be pricked with a fork.