Last night (August 23), viewers saw the former EastEnders star's first episode as Christine Mulgrew, who is a new English teacher for Waterloo Road following the school's relocation.
Christine's drinking problem and how it affects her teenage son Connor (Shane O'Meara) will be an ongoing story arc for series eight.
Brett, who bowed out as Walford's Jane Beale earlier this year, told The Scotsman: "I probably wouldn't have jumped into something so long-running if it hadn't been for the part. The part was just too good to pass up, really.
"Christine is so different to Jane. That's what attracted me. I don't know if I'd have been so eager if the character had been similar to what I've done before, but when I found out who Christine was and how she was, it was a no-brainer."
Discussing Jane Beale's 'nice' nature, she continued: "That's the hardest thing to make interesting. With someone like Christine who is full of flaws, it's so easy to get your teeth into that and have great fun with it. Playing someone who is just basically a nice woman for eight years did make me rack my brains a little bit.
"I'm always wary of being preachy, but hopefully some water cooler moments will come up around Christine. Not in a contrived way, but in an honest way as people respond to what she does.
"There are a few moments when I've come off set or out of the scene and people in the crew have said, 'My god, it does make you think about how much you drink'."
Waterloo Road continues next Thursday (August 30) at 8pm on BBC One.