6. If They're Killed Offscreen, They're Coming Back
Whether hero or villain, don't start mourning or celebrating too soon, even if they appear to have been mortally wounded, or there was simply no way they could have escaped that whole-building explosion.
If you don't see the bullet wound or slit throat, feel their body go cold or pulse stop, you shouldn't believe for a second that your favourite Whatshername isn't waiting for you a few thousand cells down the line. In fact, many films have built final-act surprises, plot shifting turns and whole sequels on the back of this trope.
The Joker's helicopter is blown up before it touches ground in 2016's Suicide Squad, but he returns to bust Harley out of Belle Reve in the final segment.
006 is apparently shot in the head in the opening sequence to 1995's Goldeneye, only to resurface an act or so later as the film's suitably facially disfigured primary antagonist (and there is a whole trope of its own belonging to the facial disfigurement of Bond baddies).
More recently, the fact that 2018's Halloween chose not to show Michael Myers' demise, backed up with two planned sequels (including the upcoming Halloween Kills), suggests that it has fallen into this trope once again - unless there are bigger shocks in store.