The actor is reprising her lead role ITV’s eponymous crime noir drama, which returns for its third run later this month.
However, Anna has revealed the dramatic toll playing such an unstable character takes on her body.
In a press interview (via The Sun), the former Brookside star she she would sometimes be sick in between scenes due to how heightened Marcella can be.
Anna said: “I tend to let it affect me once it’s a wrap on the whole season because otherwise it can get overwhelming.
“Sometimes I’ve got myself to such a state that I’ve kind of rushed into the trailer and thrown up because the cortisol levels have gone to such a high status and the body is being poisoned by the stress,” she continued.
“But you do relive it, because I give everything to her – I become her to a certain extent.”
The series two finale saw Marcella suffer a mental breakdown after unlocking a memory that she’d killed her baby while trying to pacify her.
After abandoning her former life, a homeless Marcella was later approached by a police officer with the chance to work as a shadow agent, as her family and colleagues were left to believe she had died.
The new series will pick up with Marcella in Belfast, where she has taken on a new identity, Keira, to infiltrate the infamous Maguire crime family.
As she investigates their activities, questions come to the fore about how much she’s embraced the Keira personality and left Marcella behind, as her old life risks catching up with her.
Marcella returns on Tuesday 26 January at 9pm on ITV, with all episodes then available to stream on ITVHub.