The Oscar winner had planned to treat his wife Danielle to a romantic vacation in the Italian capital once his promotional duties for Robin Hood were complete, and when his co-stars found out, they decided to join him.
Now, the break for two has become a group holiday centred around singing on the Spanish Steps.
Crowe, who fronted rock band Thirty Odd Foot of Grunts, reveals, "It's Danielle's birthday, so I was gonna have dinner with her in Rome and I told the Merry Men this and they were all like, 'That sounds like a lovely idea, Robin.' So, now I've got Little John (Kevin Durand) and Will Scarlet (Scott Grimes) and Alan A'Dayle (Alan Doyle) and their wives and girlfriends, they're all coming too.
"We thought while we're there we might just go and do a little busking on the Spanish Steps... I put it out on Twitter and we'll see how many people rock up to the Spanish Steps."