Emily Blunt Says 'Girl on the Train' Extras Thought It Was a 'Friends' Reunion

This video is unavailable because we were unable to load a message from our sponsors.

If you are using ad-blocking software, please disable it and reload the page.

In Emily Blunt's new thriller, The Girl on the Train, she plays a troubled woman named Rachel, while former Friends star Lisa Kudrow plays a friend of her ex-husband’s named Martha.

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."

WATCH: Emily Blunt Was 'Wracked With Terror' Over 'The Girl on the Train,' But in a Good Way

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."

Blunt, who welcomed her second daughter, Violet, with husband John Krasinski in June, also dished on how their 2-year-old, Hazel, is adjusting to life as a big sister.

"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.'"

NEWS: Emily Blunt Calls Out Mom Shamers: 'Women Can Be a Bit Cruel About Each Other'

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.

WATCH: John Krasinski Dishes on Baby No. 2 and Wife Emily Blunt's 'Mary Poppins' Prep

Recently, Blunt sat down with ET while promoting her upcoming thriller, and she couldn't help but marvel at how wonderful her adoring husband is.

"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.