The Australian actor's response was "absolutely immediate".
"It's not much of a decision, really, from where I come from. It's more just a lot of jumping up and down and screaming, 'Yay!'."
It's no wonder he's excited. Mendelsohn, 47, is known for playing dodgy dudes, druggy losers, and down-and-out no-hopers in films like Idiot Box, Animal Kingdom and Killing Them Softly.
He has, until now, been happy to play the supporting role, often stealing scenes, shows and movies despite rarely playing the leading man.
But that's changing. In September, he won the Best Supporting Actor Emmy for his role as Danny Rayburn in Bloodline, Netflix's dark drama about a dodgy brother returning to Florida Keys to mess up his family. He's just finished filming a role in a Steven Spielberg sci-fi movie.
And now comes Star Wars, playing another dodgy dude as Krennic, Darth Vader's right-hand man. It's something different. Something bigger. Something that can and probably will change his life.
Mendelsohn's knows this. Just a week out from Rogue One's release, it's something he's been thinking about a lot.
"I'm well aware of the level of anticipation and importance of the film. You don't know what it's going to feel like, and I still don't know," he says warily.
"I expect a lot of people will see it, but I don't know what it will be like afterwards ... we'll see if it becomes weird going out ... "
Easing Mendelsohn's concerns is something about Rogue One that he states simply and calmly, like it's already a fact.
"It really is an extraordinary film. Once you've seen it, it will make perfect sense. It really is - it's extraordinary."
That's something that's bound to please Star Wars fans hanging out to see Rogue One, the first standalone film outside of the franchise's third trilogy that lightspeeds its way back to the events leading up to A New Hope.
Fans pester him daily with questions about the film's plot, which he refuses to talk about. TimeOut gets a similar response too.
But if anyone should know, it's Mendelsohn, a card-carrying Star Wars nerd who was raised as a card-collecting superfan of the first film, A New Hope.
"I loved Han but I wanted to be Luke," he says. "I really loved all of them, C-3PO, R2-D2. I wished I had a friend backing me up like Chewbacca. I loved Princess Leia, Obi-Wan Kenobi, the whole gang, the whole box and dice. It was a very complete world."
Scoring a major role in a Star Wars film is a big deal to a Star Wars nerd. That means Mendelsohn had plenty of geek-out moments on set.
"The one that really put me into overdrive was when we were on one of the Imperial decks. There are the officers there working on computers ... I looked and one of the computers that they have, I remember these from the ones in the 70s, and the programme on that computer was exactly how I remember it."
For now, Mendelsohn is preparing for the upcoming Star Wars carnage by keeping busy. He's filmed Spielberg's Ready Player One, is working on the animated movie Larrikins, has a role in indie flick Untogether, and says he has "one or two surprise things" yet to be announced, and another two things "I'm yet to start work on".
As for the third and final season of Bloodline, in which his character Danny died at the end of season one, but still had a recurring role in season two, Mendelsohn has only this to say: "It would be remiss of me to let any cats out of the bag."
Mendelsohn's back to being secretive again. Like that poolside briefing, it's a technique that seems to be paying off for him.
LOWDOWN
Who: Ben Mendelsohn
What: Rogue One: A Star Wars Story
Where and when: In cinemas from December 15
Also: Bloodline, streaming on Netflix
