The 27-year-old is said to be besotted with Righton but she hasn't managed to put together any plans for their big day yet, reported Us magazine.
"She gets flustered when it is mentioned. She's getting ideas and trying no to stress. What matters to her most is being with him," a source said.
The 'Anna Karenina' star had previously said she felt pressured by the media to rush into their marriage and have kids.
"People keep trying to make me pregnant! I'm not ready for it. Yes I'd love to at some point but not this quick, stop trying to make me pregnant. I have been asked whether I am pregnant yet about four or five times. And I started thinking, 'Do I look like I'm pregnant ... I'm a very thin girl, I am sure I don't look pregnant!' It's the ring on my finger!"