If you were working as a lighting board operator, how would you know when to change the lighting? Read the lighting cue list.
Read along with the script.
Watch the producer.
Do what was discussed in rehearsals.

Generally spontaneous human combustion is something that still bewilders some, human combustion happens very rarely and doesn't affect the surrounding areas, so unless you want to set your self on fire, human combustion will remain spontaneous and very rare.

The thing people think is that big heads mean big brain.