"Marvel hasn't even told me what the end of this movie is," Downey told NDTV.com during an Infinity War event in Singapore.
"We don't give anyone the full script," said executive producer Trinh Tran, who explained the stars of the film "just get their part of the script."
It's not until the actors reach the set that they fully know what they'll be doing, Tran explained, saying the policy was birthed out of Marvel's calls for secrecy.
"Some days the actors don't even know what they are shooting, they are told on the set what they will shoot that day," Tran said. "Out of all the Marvel movies, this is one of the biggest kept secret films. We went in being very protective."
Avengers: Infinity War directors Anthony and Joe Russo recently issued a formal letter addressed to fans, asking them to refrain from spoiling the superhero epic for other fans before everyone has a chance to see the movie.
Marvel has long had a tradition of shrouding its high-profile productions in secrecy — its stars are well-versed in the running gag about "Marvel snipers," waiting in the wings and ready to snuff out any loose-lipped leaker before spoilers can be dropped — but Infinity War is especially covert.
That hush-hush has given rise to even more theories from fans whittling together what little information they have to postulate popular beliefs like "Iron Man dies" or "Captain America dies," but asked by NDTV if he reads such widespread speculation online, Joe Russo said online commentary doesn't influence their story.
"It's good to stay connected to the fans and be careful you don't let too much information out," Russo said. "It should not affect the storytelling. Fans spend so much time and effort, and we spend the same amount of time in making it. It's the end of a ten-year journey of a franchise; no none wants to hear the end from someone else."
With the Marvel Cinematic Universe bringing its first chapter to an end and readying its second, Downey has been at the center of talks questioning if his 10-year tenure as Iron Man is coming to its own close.
The star told Empire Magazine he's "looking forward to whatever resolution is in the cards" for his time as Tony Stark-slash-Iron Man, and whether that means shooting off into the sunset or ending six feet deep remains to be seen.
Avengers: Infinity War opens April 27.