I believe I am the Potions master at this school  This vignette
isn't part of the seven part series that starts with "Why Snape
never eats here" – it's just a reflection on what Filch,
Lockhart and Snape were thinking the night that Mrs Norris was
Petrified. However, if you read the series you will get a better
feel for the particular version of the Potterverse in which the
story is set.  There's nothing earth-shattering in this, just a
little fun with a couple of minor characters.  Chapter 1: Argus
Filch  He doesn't normally eat in the Great Hall with the
teaching staff and the students, he normally eats in the kitchen
with the house elves or takes a snack in his office – fried fish
with chips is his favourite, and Mrs Norris always has a bite of
the fish, she always has a little something from his plate to be
companionable. But Halloween and Christmas feasts are an
exception to the rule, so tonight he's put on an old and rather
mouldy looking tail coat and joined the professors for dinner.
You can't have a