JerryM
You make a good point, the base OR the chopped chillie (or pepper, onion, whatever) should do the job of helping to prevent the oil from overheating. I guess I'm just using a 'belt & braces' approach. I think provided an excess of oil is present, it is not necesssary to use both. The important thing, I think, is to cook the spice in the oil untill it JUST gets to the point where all the moisture has been driven out of the 'moisture containing ingredient' (base, pepper, whatever!), because once the moisture is gone there is nothing to stop the temperature of the oil climbing rapidly and burning all your hard work! That said, the spice must be cooking in the 'oil rich environment long enough to extract the essential oils form the spice that constitute the flavours, that's why you don't get a BIR taste if you stop the spice-frying processs too soon (ie before the moisture has gone, indicated by a reduction in 'sizzle')
Using a heavy pan helps, it's far easier to ruin the spice in a wok, even though the temperature can be adjusted more quickly in a wok, it can climb really fast, spice can go from perfectly cooked to burnt in a couple of seconds if it's not watched carefully. The disadvatage of a heavy pan is that once the pan get too hot, the only way to control things is to be ready to add more base quickly, simply reducing the gas will not have the immediate effect it would in a thin pan, BUT In my opinion you get more warning (and time to act) with a heavy fring pan.
The whole BIR thing is much more 'technique critical' than just about any other style (although chinese can be tricky!). To get the optimum taste it is necessary to stop the spice frying bit of the final dish preparation just short of burning...
When I cook a bhuna, I fry garlic, then add, pepper and onion, then tom puree, and when THEN spice, so in that instance I add the base (much less, in a Bhuna) AFTER the spice, because there is a relataively larger amount of moisture containing veg in the pan to control oil temperature.