I use a gas burner with a fairly high flame to char them as quickly as possible. Char them too slowly and the flesh overcooks. Ten minutes in plastic and peel. Do not peel under running water as that just washes flavor down the drain.
It's perhaps possible to char them too quickly. I once had a chile en nogada where they must have used a blow torch. The flesh was nearly raw, which gave it a nice crunch but lacked the roasty flavor.
FWIW, Diana Kennedy recommends brushing with oil when using a broiler, and not to use a hot oven as that again overcooks the flesh.
ETA: I wouldn't worry about bitterness. I tend to almost completely blacken the skin and they come out great. I once made a roasted habanero salsa where I left them over the flame too long -- some were completely carbonized in spots. I made the salsa anyway and to my surprise it was delicious.