McAvoy and Irishman Fassbender (Inglourious Basterds, Hunger) have known each other since working on Band Of Brothers in 2000, so their First Class double act sprang to life quickly.
"When we met up to talk about doing this, we both agreed there's no point in making these big, silly pieces of fun unless you can enjoy it, too," McAvoy says.
"As hard as it was at times - because these things are very hard to make, they are a storm happening all around you every day for six months - we made a pact to try to make each other laugh as much as possible."
In a fab montage, the pair go on a recruiting drive, in which they perch on a strip-club bed.
"That scene got exceptionally silly at one point," says McAvoy. "I don't know if I'm even allowed to tell you what originally happened. But it got pretty f---in' kinky. I'm not joking."
Was the dashing duo's bromance, uh, consummated?