Agatha Christie's Miss Marple: Sleeping Murder

Gwenda Reed:
Why didn't *we* think of that?

Miss Jane Marple:
Because you believed what he told you. It's very dangerous to believe people - I haven't for years.

Miss Jane Marple:
Good advice is almost certain to be ignored, but that's no reason for not giving it.

Miss Jane Marple:
There's always possibility of X.

Gwenda Reed:
X?

Giles Reed:
X?

Miss Jane Marple:
The unknown factor... someone who hasn't appeared yet but whose presence behind the facts can be deduced.