"I had worked with Shonda a couple of times before and she said she wanted to talk to me about some 'stuff.'
"She wasn't very specific, then she started talking about Grey's Anatomy, which was exciting."
Henderson thinks his first acting role on Shortland Street was harder work than Grey's Anatomy.
"Anyone working on that show [Shortland Street] at that pace is doing a great job just to remember all their lines. This show is definitely more forgiving. We do one episode every 10 days. There's a lot of set-up so if you don't totally know your lines in the wide shot you kind of know them by the time you do your close-ups - don't tell Shonda I said that!
"It's funny. Every now and then I'm standing at reception, I've got my clip board, I'm going through my lines and I'm waiting for action. It's the same environment as Browns Bay all those years ago."
Season 12 is well under way in the US and Henderson has a new fan base because of the famous show. But he isn't kissing and telling about whether he takes up where McDreamy left off.
Offscreen, Henderson has been hanging out with another Grey's Anatomy newbie and fellow singleton in Hollywood, Giacomo Gianniotti, who plays Dr Andrew DeLuca.
The pair were both dateless on Valentine's Day, so ended up at Henderson's house, then a neighbour invited them over.
Henderson was asked to make pavlova, but - sacrilege - he did not know how. Apparently guests were pleasantly surprised by the pav Gianniotti whipped up.