When it comes to the Marvel Cinematic Universe, secrecy has proven to be vital throughout the years -- so much so that actors are normally terrified to confirm their involvement with a forthcoming project early on, fearful they may accidentally slip something significant ahead of the film's premiere. However, in recent years, it appears as though the Marvel snipers have become rather lax, as returning cast members have been noticeably captured accidentally letting details slip during interviews. With that in mind, we take a look at some of the biggest spoilers revealed by the Marvel cast themselves -- here's mainly looking at you, Mark Ruffalo and Tom Holland!RELATED: Avengers 4 Will Resurrect a Doctor Strange Character, Too

The Guardians Cast Spoilers Their Returns in Avengers 4:

It was always expected that the events of Avengers: Infinity War would be altered in one way or another, bringing back the characters we lost at the hands of the Mad Titan, Thanos. After all, with most of the Guardians gone, a third film seemed pretty pointless. Thankfully, the Guardians themselves had no problem confirming that they'd be back in May's Avengers 4, including Zoe Saldana's Gamora, who Thanos famously sacrificed in order to retrieve the soul stone, as well as Dave Bautista confirming a return as Drax. Were their comments a huge surprise? No. Unfortunately, though, in the case of Saldana, her comments came before Avengers: Infinity War officially hit theaters, meaning fans knew not to expect her death to stick, especially since she later went on to confirm she'd be back for Guardians of the Galaxy Vol. 3 -- which has since been put on indefinite hold.

Sebastian Stan on a Potential Avengers 4 Cameo:

Having made her debut in this year's Ant-Man and the Wasp, it seemed safe to presume Michelle Pfeiffer would be involved in the next Avengers film in one way or another. Of course, just because it was expected didn't mean it was confirmed. Until Stan opted to seemingly do so, of course. During an appearance at Ace Comic Con, Stan talked about a scene that they filmed which appears to be for Avengers 4. Despite doing his best not to say too much, the actor still managed to give away a tiny bit of news. "You look around and you just saw everyone from Samuel L. Jackson to Michael Douglas to Michelle Pfeiffer. Everybody was there."

Yes, both Michael Douglas and Michelle Pfeiffer will be back for Avengers 4.

When Tom Holland Accidentally Spoiled Peter's Infinity War Fate:

During the opening weekend for Avengers: Infinity War, which is easily the biggest Marvel Studios production to date, Marvel Studios president Kevin Feige decided to surprise some fans at a showing along with a few special guests. Not only did Feige bring along directors Joe and Anthony Russo, but he also brought out Spider-Man actor Tom Holland. As the crowd erupted in cheers taking in the actor's presence, Holland famously reached for a microphone and shouted "I'm alive!" to the crowd. The problem? They had yet to see the film and had no idea that Peter would turn to ash in Tony's arms at the end. A bit early there, Tom!

Thankfully, the crow appeared to take it all in jest and still went on to enjoy the film despite the actor's minor slip-up.

Star-Lord Apparently Manages to Meet Shuri:

Sometimes, spoilers come from the most surprising sources. In this case? It's none other than Shuri actress Letitia Wright. During an appearance at MCM London Comic Con earlier this year, Wright made a comment that seemed to suggest the duo would come to meet in Avengers 4. "Chris Pratt was genuinely a great person. He would like give me bible scriptures out of nowhere and then do a dance and leave. He’s great. I love Chris Pratt," she said.

Of course, as fans will note, the duo never crossed paths during Infinity War, and unlike Star-Lord, Shuri wasn't a victim of the snap. How the two will come together remains to be seen, as does how the film will go about reviving the killed-off heroes. However, one thing is for sure: Star-Lord more than likely has no idea what he's up against with Shuri.

Gwyneth Paltrow's Willingness to Spoil Avengers 4:

Pepper Potts & Tony Stark

Paltrow, who has been a part of the Marvel Cinematic Universe since the beginning, had been noticeably absent in recent films. That is, until she finally made a return in Spider-Man: Homecoming. Since then, Paltrow has been quite open regarding her return to the franchise, not only posting (non-spoiler) behind the scenes photos, but also posting straight up spoilers from set that seem to confirm she'll finally suit up in the fourth Avengers film. That isn't all, though. Paltrow has also previously teased a child for Tony and Pepper in the film, something that had been previously speculated following Avengers: Infinity War.

NEXT PAGE: Tom Holland and Mark Ruffalo Can't Stop Spoiling Marvel Movies

Tom Holland Spoiled a Scene in Spider-Man: Homecoming:

If there's a spoiler that is revealed courtesy of a Marvel actor, chances are the person responsible is either Holland or Ruffalo. In this case, it was the former, who was trying his best to describe the stunt work that went into making Homecoming. For the most part, Holland was doing a pretty good job at detailing the extensive stunt work, explaining what he was able to do himself, and what his stunt man was required to perform -- such as the helicopter scene in the film. "There was one time where they dangled my poor stunt double under a helicopter and dumped him into a lake," he said before immediately covering his mouth having realized his mistake. The problem? This interview took place prior to the film's release.

Over the past year or so, Marvel Studios has gotten in on the fun, using Holland's willingness to share news with his fans as a way to unveil the first poster for Avengers: Infinity War, as well as the title for the follow-up to Homecoming. It's a smart move that has paid off for the studio in recent months. However, it's probably best they continue to pair Holland with Doctor Strange actor Benedict Cumberbatch during interviews for Avengers 4. You know, just to be safe.

Mark Ruffalo Accidentally Streams the Audio of Thor: Ragnarok

Thor Ragnarok Bruce Banner

Sometimes, the actors don't even need to speak in order to spoil anything. Case in point? Ruffalo's little -- well, major -- accident during the world premiere of Thor: Ragnarok. Being one of the few Marvel actors that is incredibly active on social media, Ruffalo tries to make sure fans get to experience the premiere along with him. Unfortunately, though, it also leads to unintentional spoilers.

After being seated at the premiere, Ruffalo seemingly forgot he'd left his Instagram stream up and continued to stream from inside the theater. Which, sure, the inside of his pocket wasn't necessarily a spoiler. The audio for approximately the first fifteen to twenty minutes of the film, however, certainly were and famously led to the actor getting a talking to from Marvel Studios.

Mark Ruffalo Spoils the Ending of Avengers: Infinity War

Ruffalo means well, he does, but he's also incredibly open without even trying to be sometimes. The best example of this? The interview he did last year to promote Infinity War, where he just casually said that everyone was going to die in the end. "Wait 'til you see this next one!" He exclaimed. "Everybody dies!" The comment led to a disapproving "dude" courtesy of co-star Don Cheadle, and Ruffalo tried his best to walk the comment back. This being the internet, though, once it was out there, there was no going back.

Sure, it was a bit of an exaggeration, as not all of our heroes died. However, it was pretty dang close, and the actor has since gone on to admit he had unintentionally spoiled the ending ahead of the film's release.

RELATED: Avengers 4: Frank Grillo Has 'Confirmed Nothing' About Crossbones' Return

Next up for Marvel Studios is the incredibly secretive Avengers 4, which premieres May 3, 2019.