"We couldn’t really prepare the way we normally prepare," Diehl explained. "Normally we go to, I don't know, Valentino or Armani, and say we are shooting a cover and ask if they can give us some clothes [to borrow]. For this, I had to go into stores incognito, so to speak -- not like anyone would know what I looked like anyway -- and buy stuff. That was actually what was fun."
But that wasn't the extent of the efforts to keep the shoot under wraps, which took "James Bond-esque precautions."
"The secrecy, I have to say, had a lot to do with being discreet and shopping," Diehl said. "We talked to no one. Literally, the people here at Vanity Fair did not know. Three people in the fashion department knew, including me."
While a lot of thought had to go into Caitlyn's look, to make the best first impression
, Diehl said it was clear the looks would have to be elegant, and therefore chose to focus on more timeless styles rather than cutting-edge fashion.
"Coming from a very fashion-forward family, we sort of thought that the nicest thing to do was to focus on style over fashion," Diehl said, referring to Caitlyn's famously fashion conscious and trend-setting immediate family. "Nothing too bling or too shiny. Kind of keeping it more in the style arena rather than hardcore fashion arena."
Diehl also opened up about the various inspirations she and Caitlyn used to prepare the ensembles, saying that they looked at everything "from Lauren Bacall to The Thomas Crown Affair."
"And just vintage photographs of old Hollywood stars," Diehl added. "A couple of Jackie Kennedys are never wrong."
As for Caitlyn's eventual style, it turns out the 65-year-old former Olympian has remained fit and athletic, meaning she's "easy to dress" and everything looks good on her.
"We had basically a three or four hour fitting. I have to say, out of that three or four hour fitting, there were only two things we did not like. Everything looks good on her. That just doesn't happen," Diehl revealed.
In the end, the designer shared that it was Caitlyn's confidence and enthusiasm that really made the experience something special.
"She was comfortable, confident, and has absolutely no reason not to be," Diehl recounted. "It was probably the most exciting thing to see -- there was real joy in her feeling comfortable and happy in her skin. It was amazing."