Would my character have done that?
This is not a question of someone acting out of character. The action may well fit the personality, but is the incident itself plausible in the given context? Gangster A might be quite capable of murdering villain B i.e. it is in character, and it is necessary for the plot, but the reason has to hold water.
This is why a decent editor is priceless. Not only do they check for any grammatical, typo and format problems (which are incidentally also vital factors) but to see that the tale itself doesn't have logical flaws or inconsistencies.
For instance, in a recent TV showing, several mass murders occurred behind locked doors in London houses. Police baffled.
Later in the episode it turns out the murderer, a builder, had renovated the buildings and built in hidden passageways and hiding places. Now, some Met officers may not be the sharpest knives in the cutlery drawer but they are meticulous, and in such a case it is totally inconceivable that they would have overlooked hidden doors or hollow walls.
Having a character strike a match on a bar of soft soap is more likely.
Suspension of disbelief is one thing, sloppy writing is another.
'Write about what you know' was always the advice given to tyros because it helped avoid making howlers. Knowledge helps stop your actors from mowing the enemy down at 300 metres with a 9mm pistol or stepping over an exploding grenade and only getting singed leg hairs.
Deus ex machina or 'with one giant leap he was free' solutions have always been scoffed at by serious readers and writers but if recent reading is any indication, the increase of Indie authors who have no editors means they are making a comeback.
The inexperienced might ask, 'what does it matter?'
It matters more than before if you are in the business of trying to sell your wares. Criticism abounds. People are rapidly learning that intelligent reviews are worth their weight in sales rather than the piles of fluff of the 'This is the best book I've ever read' type. When a reviewer points out several “Oh, come on!” moments then a potential buyer will move quickly on. There's a ton of choice nowadays.
So, don't have your circus stunt diver walking away after a 'double back with half twist' into a wet handkerchief from the 20 metre board.
If you want your readers throwing up their hands with delight and not despair then check your plausibility. And get someone to go over the grammar, spelling, typos and format while you're at it. Be professional.