Often you'll see an Italian opening film credit, "con le participazione di" (literally: "with the participation of"). The correct English equivalent would be "special guest appearance by", and it typically means that a star-calibre actor has agreed to take on a smaller role for any number of reasons: they're doing the role as a favor, they needed the money but everyone was too polite and respectful of their talent to mention it, they liked the script, etc.
It may have been in this case that Rossano liked the story and the script, and if so, you can't help but applaud his taste in genuinely heart-felt and affecting love stories ... in fact, in this case, some of us are surprised that this sweet film DIDN'T make it to the U.S. - it should have.
"Bersagliere" is an odd word: to bomb, to harrass, but it also means "fool". The character in question is both: in the Italian army, and also a bit of a "fool" -- or, for all we know, it could have been the character's last name! "Ragazza" tends to interpret more as "intended", than "dating companion".
Salvatore is a young, charming conscript in the Italian army, on maneuvers. He spies a beautiful woman in a swimming hole and pursues her - it's love at first sight between the two of them, and in between maneuvers they furnish their own lovenest in an abandoned cottage and plan their marriage. Salvatore, unfortunately, drowns in a nearby lake before the marriage can take place.
Soon after the funeral, he begins appearing to her as a ghost and they are trapped in a sexually frustrating mismatch of flesh and spirit ... they both want each other desperately, but lovemaking tends to be difficult when one partner is able to walk through walls! The "fool", in this story, could refer to either Salvatore -- whose charm lies in his irreverence (one of his most endearing props is a silly feathered hat), to the fools that surround them in their lives, or to the only person other than his beloved able to see him - the village idiot. The final resolution to their problem is as sweet and heart-rendering as it is inevitable. It's an enjoyable movie.
So -- how does Rossano fit into this story? Rossano's character is a rather pompous, intellectual nerd who doesn't believe for a moment in life after death, or in anything metaphysical, for that matter -- and his "fool" - complete with the thick tortoise-shell glasses he keeps pushing up his nose - is a delight to watch ... he doesn't often play these sorts of characters, and certainly appeared, in this, to have enjoyed the role. The frustrated maiden has complained to him that Salvatore continually interferes with her becoming romantic with any other man (she is, after all, still a full-blooded Italian signorina!) -- he pooh-poohs the entire idea and promises to prove to her that every incident she perceives as a manifestation of Salvatore's jealousy can be explained away rationally as a natural phenomenon. To that end, they arrange a "romantic evening" at the cottage. Salvatore, of course, wins the eventual battle of wits and sends our hero running off into the night in a state of fright, but not before he has turned in a wonderfully un-Rossano-ish performance in a sweet, memorable love story.