The 17-year-old almost decided on an alternate career after he lost out on a role in an unnamed film - he figured he'd later step behind the camera as a director instead.
The star, who was then 12, reveals he was "sick in bed and almost considering not even acting (anymore)" at the time he was due to submit his audition tape for the Netflix series.
The low key try out also included a chat with the show's casting directors but Finn still couldn't get excited about the possibility of being cast.
"They just kind of pitched me the show - I was, like, 12 - and we talked about all the movies it was based on," he shares.
He went on to land the role of Mike Wheeler but admits he had limited expectations for the sci-fi project, which was created by Matt and Ross Duffer.
"We just thought we were filming this secret thing that no one knew about," he shares. "Which we were. No one knew what we were doing. Netflix was kind of hands-off. We thought maybe it would become a cult classic, and we'll come back to it in 30 years and be really proud of doing it."
Meanwhile, "Stranger Things"' season three audience smashed previous viewership records for the platform, with 40 million households streaming the show. Filming of season four, which was taking place in Atlanta, Georgia, was halted due to the coronavirus pandemic.