The actors sign the stickers and then Topps is free to use them on any card they choose. When Harrison Ford signed, 10 of the stickers were used for the Stars Wars 30th anniversary set. Now, Topps is coming out with an Indiana Jones set and guess what? There will be Harrison Ford cards. The stickers give the manufacturer more flexibility with how the signatures are used. Personally, I don't like them because I think the sticker detracts from the look of the card.
I purchased my first sticker auto a couple of weeks ago. It's a Vince Neil (Motley Crue) auto from the Donruss Americana set. It's hard to see the signature, because the area behind the sticker wasn't whited out. I also noticed something odd. The signature didn't look quite right, so I went looking for other types of autos that he signed. The singer signs vertically and Topps put the sticker on horizontally!
