The cast of To Rome With Love were only given sections of the completed screenplay since the movie featured four vignettes, but Gerwig admitted that curiosity got the best of her.
"I stole the script from the mega trailer, so I actually did read [it all]," Gerwig declared to U Interview News.
Gerwig went on to offer a tongue-in-cheek defence of her behaviour: "I did that at the end of our shooting, because I actually do like just knowing your section, I think it allows you to stop intellectualising how you fit in and just be in the scenes... [I was] super entertained [by the movie]."
The actress previously suggested that working with Allen was her "favourite thing in [her] life".
"He's my favourite filmmaker, my favourite writer," Gerwig said of Allen. "I've adored him ever since I was able to read and watch movies. Doing this was kind of the best thing ever."
To Rome with Love is now playing in US cinemas on limited release.
Watch a trailer for To Rome with Love below: