To be fair, it is technically correct to say that Nicolas Cage is playing Spider-Man in 'Spider-Man: Into The Spider-Verse', it's more accurate to say he's playing one of several Spider-Men.
Quite simply, it's in the title. There's going to be multiple iterations of the web-slinger and Nicolas Cage voices just one of them - Spider-Man Noir. This version of the character was set in New York in 1933 and followed the same general beats of the story - Peter Parker's an inexperienced reporter who becomes imbued with spider-like powers and finds himself battling an evil villain by the name of Norman Osborn, also known as the Goblin.
The key difference with Spider-Man Noir compared to say, 'Spider-Man: Homecoming' is that not only does he NOT wear the blue-and-red outfit we all know, this particular Spider-Man uses firearms and has no problems killing people to get his way. Think Spider-Man crossed with The Punisher in '30s New York and you're on the right track.
It's hard to know precisely how the different iterations of Spider-Man will come together in the animated film, but given that it's just that - an animated film - there's obviously going to be more over-the-top explanations and so on for certain things. Nicolas Cage is no stranger to comic-book roles, having almost played Superman once upon a time and recently spoke about his desire to play Joker.
'Spider-Man: Into The Spider-Verse' arrives in Irish cinemas on December 14th.