One of the most important jobs of any actor involved in a popular franchise is not talking about what they're working on. It's got to be difficult to walk around with those big secrets and not spill anything while also doing all the media that these movies entail. It turns out, that some of them do end up revealing those sweet, sweet spoilers when they're not supposed to. Taran Killam is married to Cobie Smulders who plays Maria Hill in the MCU, and while her role in Avengers: Infinity War wasn't very big, knowing what happens to her reveals a great deal about the end of the movie, and she told her husband exactly what happened. According to Killam...
I remember when Cobie first told me she was auditioning for the first Avengers, just that was enough to get me so excited. [I was like] 'Do you want me to read with you? We should rehearse again, right? Let's do it one more time. What's she going to wear?' She actually snuck me a little illegal photo from the screen test. She did a screen test with about four or five other actresses. Infinity War was the first time where she let me know what was going to happen. Her tag was added on later in the schedule. She's like, 'Yeah, I'm there and then I disappear?' I was like, 'God, what does this mean? What does this mean?'
So Cobie Smulders told her husband what happened to her character in her one scene, though apparently without the actual context of what came before. Taran Killam is clearly a fan and so he almost certainly had at least a basic familiarity with Thanos and the Infinity Gauntlet, so figuring out what the vanishing meant might not have been that tough a leap, though certainly there's no guarantee that the disappearing wasn't caused by some other source, so the exact circumstances were apparently still up in the air based on what he told the Fandom Files Podcast (via SyFy Wire).
On the plus side, while Cobie Smulders gave up some spoilers to her husband, he was able to keep his mouth shut so that nobody even knew that he knew until long after the film's release. On the flipside, you have somebody like Tom Holland, who has become as well known for his giving up spoilers as he has for playing Spider-Man. He tends to make his comments directly in interviews for the world to hear. It's certainly never intentional, but it's hard to talk in circles without letting little details slip out. It's probably also difficult to live with somebody without being able to talk about your work. It's honestly surprising that Smulders made it this far without giving her husband details before.
It's unclear if Taran Killiam has been told what happens Avengers 4. We probably won't actually know until after that movie comes out on Blu-ray.