Here are some reviews on Possession with specific comments about Ms Ehle.
From The Courier Journal, Judith Egerton
For many moviegoers, Ehle's character will prompt an instant flashback to Meryl Streep in "The French Lieutenant's Woman." Ehle, with her heart-shaped face, hooded eyes and fair skin, could be Streep's younger sister. And in several scenes, she wears a emerald-green velvet cloak similar to Streep's hooded wrap in "The French Lieutenant's Woman," a movie that also moved back and forth between a modern affair and a tragic Victorian romance.
From Variety.com, Todd McCarthy
The superb Ehle is the picture of 19th century English beauty as LaMotte, porcelain complexioned and rounder of face and body than the contemporary model, as well as sharp-eyed and well spoken. Headey compels sympathy as LaMotte's needy, anguished lover, and other supporting perfs are well judged.
From Culture Vulture, Arthur Lazere
Ehle (Bedrooms and Hallways, Sunshine) is engaging as LaMotte, projecting both intelligence and alluring sexiness, making Ash's passion for her seem fully understandable. (Her resemblance to her radiant mother, actress Rosemary Harris, is palpable.)