Level 1: Correcting
This is basic copy-editing: spelling, punctuation and grammar are corrected, and consistent styling is imposed. Which is correct, realise or realize? Both are acceptable but a good editor will make sure that -ise or -ize endings are consistent throughout the text. The editor can work at Level 1 without consulting the author.
The police should have acted sooner to diffuse the situation.
The writer has confused diffuse (to spread over a wide area) with defuse (to reduce tension). The corrected version:
The police should have acted sooner to defuse the situation.