I've asked myself that very question sometimes. I think I would just say "hi!" in the nice aknowledgement of "hey, I recognized you!" but without driving them crazy. Unless they are deviant friends. Then I would glomp them. Haha.
Depends which artist. I'm a bit of a sociophobe to begin with so depending on the person I might clam up. But if it's one of my faves here on DA, who I talk to more often, I'd probably be all "OMG HI YOU HAVE A FACE NOW AWESOME!" LOL
Then there's one of my favorite YouTubers, from whom I am demanding Epic Man Hugs from if I ever see him on the street, since he said any of his fans can feel free to demand them of him if they see him. XD
Since he's dead, he would be considered a zombie and I would have to end it. :3 1: Leonardo De Vinci (Not just for his art, man. <3)
2: Frank Frazetta (He was fucking amazing. Sad he's dead D8 . Great at anatomy.) 3: John Findley (Google Tex Arcana and read his badass comic. He's alive so no zombie <3) [link] His main web and his comic [link]