The exact procedure for appointing electors varies by state, but in most (all?) states the electors are nominated by a party. For example, in Michigan, the Republican electors were nominated at the state convention in late August. The people voting at the convention were county delegates; county delegates were chosen by vote at a county convention a few weeks before; the people at the county convention were precinct delegates and incumbent elected officials; the precinct delegates were elected back in May. The elector from my district is a 70ish retired white guy from Oakland County who has never held elected office other than precinct and convention delegate. It sounded from the remarks of his supporters like he came from a blue-collar background and had been apolitical for much of his younger life, but had been a tireless volunteer since becoming politically active.
If Trump does something sufficiently heinous and notorious between now and mid-December, or if he's actually dead, it's possible that some, most or all of the Republican electors could defect, but if they do so, they're more likely to vote for some other Republican than for any Democrat. If not all agree, that could pass the election to the House. There again, a Republican-controlled House is unlikely to choose Clinton; although it's possible as some sort of brokered deal (maybe keep Clinton as president but with Pence or Ryan as vice-president, for example).