What are the most important factors when making fried fish?
There are three important factors. One is using only wild-caught fish, preferably cold ocean species. Two is not to overuse the cooking oil; make sure you change it regularly. And third, cook for a short time at a high temperature, sealing in the juice (and blocking out the excess fats).

What is the biggest mistake bad fish and chips places make?
Using old dirty oil with cheap, fresh water fish that don’t have clean flavor, and sometimes even have a muddy taste.

Why do some recipes add ingredients like beer or milk? What’s the difference?
Beer is used for its carbonation rather than its flavor. It helps make a light batter. As for milk, people use it to remove the fishy flavor and help tenderize the fish. But it usually turns out soggy and bland. However, good fresh fish doesn’t need either.

Are there any fish that would be bad choices?
Intensely farmed fresh water fish which are grown using antibiotics, growth stimulants, antifungal agents, and food made with ingredients of dubious quality. You are what you eat, right?

What would you recommend drinking with fish and chips?
I would recommend eating fish & chips with any good lager beer or a Sauvignon Blanc, although the beauty of fish and chips is that it goes well with pretty much anything, even a glass of cold milk—my favorite when I was a kid.