The actress, who plays alcoholic English teacher Christine Mulgrew in the school drama, told The Sun that she learnt how to pretend to be drunk by observing her co-star.
"I've watched the best at playing drunk - Steve McFadden," said Brett. "I observed for years how he did it in EastEnders.
"I also try to remember the old adage that Michael Caine said, 'You don't play drunk, you try and play sober'."
Brett, who played Walford's Jane Beale for eight years, described inebriation as "probably the most difficult thing to pull off convincingly as an actor".
"Luckily [Christine] is not falling down drunk, which is actually easier," she added. "It is more about her mentally rather than playing 'I'm drunk' right now."
As well as visibly struggling with a drinking problem, Brett's character has a turbulent relationship with teenage son Connor, played by Shane O'Meara.
"What's really interesting about Christine is how she copes with the alcoholism - it makes her a really big manipulator and liar," she explained.
"She uses her son as a crutch, and they have a very co-dependent relationship."
Waterloo Road airs on Thursdays at 8pm on BBC One.