"I just love her. I do. I've had a massive crush on her for years," Marsden gushed prior to Mirren's appearance onstage.
"I was on a flight with her once to Rome, and when I got off the flight I followed her to baggage claim and in a very voyeuristic, creepy way, I took a photo of her while she was walking," he admitted. "I was avoiding seeing her backstage because I didn't want her to know I was stalking her."
Marsden was last linked to British singer Emma Deigman, aka Edei, while Mirren has been married to director Taylor Hackford since 1997. The actress called Hackford the "love of [her] life" during an appearance on the Today show in March.
"The great thing about finding a partnership later on is that you recognize it -- the partnership level of this relationship -- as much as the love and the lust and all the rest of it," she said.
"It takes place in a futuristic theme park where paying guests can go and immerse themselves in a virtual reality, essentially, of a wild west town that's inhabited by robots that are virtually indistinguishable from human beings," the actor said. "You can go and live out your most adventurous fantasy -- or your darkest fantasy, or whatever -- without any rules or repercussions."
Westworld airs Sundays at 9 p.m. ET, and was renewed this month for a second season. Marsden will also star in the movie Shock and Awe with Jessica Biel, while Mirren is slated for Collateral Beauty with Will Smith.