For the first time, Thompson has spoken out about that painful period, beginning with the end of her six-year marriage in 1995.
It's "all blood under the bridge," she said. "You can't hold on to anything like that."
"It's pointless. I haven't got the energy for it," she went on. "Helena and I made our peace years and years ago."
The pain of Branagh's betrayal was useful in at least one way: Thompson said she channeled the experience into her Love Actually role, in which her character discovers her husband may have been cheating.
"I've had so much bloody practice at crying in a bedroom, then having to go out and be cheerful, gathering up the pieces of my heart and putting them in a drawer," she said.
Thompson and Branagh married in 1989, two years after they met. Branagh and Bonham Carter struck up their affair after meeting on the set of Marry Shelley's Frankenstein in 1994, and stayed together until 1999.
Looking back, Thompson said she can see why Branagh would be attracted to both women.
"Oh, we are" alike, she said. "Being slightly mad and a bit fashion-challenged. Perhaps that's why Ken loved us both. She's a wonderful woman, Helena."
Both women have since moved on: Thompson married Greg Wise in 2003, who she met while filming Sense and Sensibility in 1995, a relationship that helped pull her out of a spiral of depression after the breakup of her marriage with Branagh. Bonham Carter has been with director Tim Burton since 2001, and they have two children.
They've also starred in several films together: in Howard's End in 1992, and the Harry Potter series from 2007. Branagh also appeared in Harry Potter, but in the second film, before either actress joined the series.
[The Sunday Times]