The Trainspotting, Boardwalk Empire and Gosford Park star admitted she was taken aback to see the transformation in her character Anna Dean in The Victim.
The show flashes back from a High Court case, in which the Edinburgh mother is accused of conspiracy to murder, to the aftermath of a vicious Halloween night attack on a bus driver in Port Glasgow.
Glasgow-born Macdonald stars opposite John Hannah and James Harkness in The Victim, a four-part legal thriller being shown on consecutive nights this week on BBC One.
Viewers are left in the dark as to whether or not Harkness's character Craig Myers, the man targeted online by Dean and subsequently attacked in his own home is the notorious child killer Eddie J Turner.
The Victim has echoes of the 1993 murder of British toddler James Bulger and the fact that both his killers, who were then aged 10, have been living anonymously with new identities since being released from life sentences.
Speaking after a Bafta Scotland screening of the first episode, Macdonald said: “This was the first time I’ve seen any of it. I’m still a bit freaked out.
“It was interesting watching it to see how different Anna is in the family home or in the courtroom. She is entirely flawed and has all these different layers. I’m really glad it worked."
Macdonald has admitted she took on her "darkest ever" role as Anna Dean,
“I wanted to play someone who was quite unlikeable. It didn’t weigh heavy on me. It was all on the page. It was easy to play the scenes as they were so well written.
"When you first meet my character you’re immediately on her side, but your loyalty shifts away from her in places, as she behaves in unlawful ways and at some points is maniacal in striving for what she believes is the truth.
“It’s almost like a mental illness which makes actors able to do what they do, and behave in ways that someone has written for you.”
Speaking previously during the filming of the drama, Macdonald said the character of Anna Dean was a far cry from the “goody two shoes” roles Macdonald concedes she is often cast in.
She said: “It’s always down to the character for me. She really appealed to me because she is so unappealing. I found her quite abrasive and unlikable in places. It’s a very tragic story and just seemed very human. She doesn’t behave impeccably.
“The most awful thing has happened to her many years before and she’s never got over it. Her grieving process has nowhere to go and the story is about her desperate attempt to make it right for her son.
“She can’t get past the fact that this person has served so little time and is able to get on with their life. It has turned her into a kind of crazed person. She’s got absolutely no respect for the police and has lost faith in them. She’s gone off on her own and she thinks she has come very close to knowing who this man is now.”