On Monday's Tonight Show, Blunt recounted how Kudrow's character was originally named Monica, and having characters with those names (especially when there’s a Friends alum involved) led to some hilarious confusion on set.
"We were shooting in Grand Central Station… and we had about 300 extras," the 33-year-old star told Fallon, laughing. "We had actual commuters coming into Grand Central who heard on the loudspeaker, 'OK guys, we're gonna roll on Rachel!' And then they see Lisa Kudrow walking on and then they hear, 'Guys, we're gonna do a close-up on Monica!' And I saw two people have an aneurysm!"
"[Extras] were like, 'It's the Friends reunion!'" she continued. "They were so excited! [And then] disappointed."
According to Blunt, The Girl on the Train team ended up changing Kudrow’s character’s name to Martha because they "were sure it would get sort of a laugh in a test screening."
"We were concerned Hazel would be really disinterested," Blunt admitted. "[But] she quite likes her."
In fact, Hazel is already trying to make Violet as comfortable as possible (even if she does so in a hilariously uncomfortable way).
"I turned my back for a second yesterday and I went to the kitchen to make us some lunch and I came back to discover Hazel just dragging Violet off her play mat. Just gently sort of just dragging her," she recounted. "I just said 'Ooh, Hazel, I don't think we want to do that.' And she goes, 'I just want to get her in a better spot.'"
And while Violet is likely still quite a few months away from saying her first words, it turns out Violet had picked up her mother's British accent -- for a while.
"Hazel had my accent for a really long time and would say things like 'wot-ter', which I was really proud of," Blunt shared. "Then, the other day she goes, 'Can I have some waaaaater?' And I went, 'It's wot-ter,' and she was like, 'No, it's water,' and I was like, 'No!'"
After their interview, Blunt and Fallon faced off for an awesome installment of "Box of Lies," where the acclaimed actress manage to pull a few fast ones over on her opponent, but not without getting fooled a few times herself thanks to an "easily pleased" audience.
"He is my perfect man, you know, he is funny and warm and bright and confident and a sort of emboldening person to be around," the actress said of Krasinski, whom she's been married to for six years. Check out the video below to hear more.