The actress's character Jane Beale last appeared on screen in January, when she departed Walford to take on a new job in Cardiff.
Brett initially planned to take time out from acting following her EastEnders exit, but she has since joined the cast of Waterloo Road as English teacher Christine Mulgrew.
Speaking on Lorraine today (August 22), Brett explained that the opportunity to join Waterloo Road was too good to turn down.
Pictured: Laurie Brett on Lorraine.
Brett commented: "I was going to take a break and then Let's Dance asked me to do Comic Relief with Tameka [Empson]. We did that, and at the same time my agent rang and said, 'Look, this part in Waterloo Road's coming up, it's a great part and I think you should be seen for it'.
"It was the first job I'd gone for coming out of EastEnders, [so I was] not really thinking I would get it, and I did. It was just such a great role and so different from Jane. The opportunity to do it was just too much to resist, really! It was just such a great thing. I'm so lucky to be able to walk out of one big job, straight into another."
The 43-year-old added that she is aware of how much Jane's ex-husband Ian is currently struggling on screen in EastEnders. The long-standing character recently suffered a breakdown and spent time living on the streets.
Brett said: "Bless him. I just saw that apparently he doesn't want to see Jane, and that's kind of heartbreaking!
"It was a huge thing, leaving EastEnders. It was very frightening and so to leave Adam [Woodyatt] and all the people that I'd been working with for eight years, it was a very scary time. Jane and Ian had kind of gone round in circles, but the door has always been left open."
Meanwhile, as Brett's Waterloo Road character Christine is an alcoholic, viewers will see that she has a turbulent relationship with her teenage son Connor, played by Shane O'Meara (pictured above).
Brett explained: "Essentially, Christine's a very sad, lonely woman inside, and there's a huge reason why she is the way she is, but that comes out after Christmas and New Year in the second part of the series.
"What's really interesting about her is how she copes with the alcoholism - and it makes her a really big manipulator and liar. She uses her son as a crutch, and they have a very co-dependent relationship.
"[Connor is] played by the lovely Shane, who's going to be such a heartthrob. He's like a young John Travolta, he's going to have all the girls after him! And he's lovely to act with and we've got a great chemistry."
Waterloo Road launches its new series tomorrow (August 23) at 8pm on BBC One. Get the Inside Soap magazine on your iPhone or iPad