While shadowing a group of photographers for his new documentary Teenage Paparazzo, Grenier said that he somehow found himself snapping pictures of Shields, who is married to Entourage's executive producer Chris Henchy.
"She had no idea I would be there... I had no idea I would be there," he told PopEater.
He continued: "When you are getting accosted like that, it's hard to see very much. It's like a dream at times even in normal circumstances. To see me amongst them must have added to the surreal-ness of the situation."
Grenier said that he had not seen Shields since the incident, but he is looking forward to discussing it with her when they next meet.
The actor added that he decided to finish the documentary after an altercation where his foot was injured by a rush of paparazzi that were trying to take photographs of Desperate Housewives star Eva Longoria.
"I quit after that. It was too dangerous. I didn't want to risk my feet. I learned it was not that easy. There's a certain skill, and I don't have it. It's a brutal job and very stressful."
Teenage Paparazzo premiere tonight on HBO.