Weisz does a brilliant job of portraying a woman who has to hide the fact that she’s falling in love while Farrell is “acting out” a description of how he would keep their love alive “even if we didn’t have kids”. Ferrell has the advantage of speaking the monolog, and Weisz is just brilliant in the subtlety of her facial reactions and body language in silent response.
At the end of the monolog, Ferrel says, “I love… my wife so much… I could die for her.” He turns his face away from her, and reaches his hand back to take hers. Again - wordlessly - Weisz looks downward, blushingly and smiling, then instantly realizes the danger of exposing her honest reaction, purses her lips and turns her eyes upward to the ceiling of the room. Ferrell turns back to face her and kisses her awkwardly. Weisz, brilliantly again, plays right on the the line of wanting to express love for him, yet fearing being discovered by Loner Leader. She smiles somewhat weakly and takes a sharp breath in through the nose, as if she were choking back tears. It’s a spectacular example of really fine acting. Even before that final moment of the scene, Weisz’ reactions are beautifully acted. I loved her eyes when Ferrel first invents their kids.