There are going to be a lot of characters in Avengers; Infinity War. There are so many of them that it can be difficult for a fan to keep them straight. However, it turns out fans aren't the only ones who get confused. Sebastian Stan recently admitted on social media that when he sees some of these Infinity War characters pop up in merchandise, he has no clue who they are either.
As an example, Sebastian Stan posted an image to Instagram of a Funko Pop for the Marvel character Corvus Glaive. He's a part of the official Avengers: Infinity War line of Funko Pop figures, but you wouldn't know that if you asked Stan, who added the caption that he had no idea who this guy was.
It would appear to be an example of just how massive the cast of Avengers: Infinity War is. Everybody is really excited by the potential of which characters will get to interact together in the massive team-up movie, but this is an example of two characters that apparently will never cross paths. If you were looking forward to an epic throwdown between the Winter Soldier and Corvus Glaive (hey, somebody probably was) you can apparently forget it, because Bucky Barnes never lays eyes on this guy.
The other reason Sebastian Stan probably doesn't know this character is that his knowledge of the movie itself is probably fairly limited. One of the precautions that studios like Disney often take to reduce the risks of accidental leaks is that they only show the actor the portions of the script they actually need to see. Since Sebastian Stan doesn't meet this character in Avengers: Infinity War, he probably hasn't seen the portion of the script where the character does appear.
For the record, Corvus Glaive is a member of Thanos' Black Order. He's the husband of fellow member Proxima Midnight, and we saw her in the first official trailer for Avengers: Infinity War, so we can expect that her other half will be along as well. That's about the extent of what I know that is more than Sebastian Stan knows, because with the exception of a Black Order statue that was revealed at the D23 Expo last year that includes the character, essentially nothing else has been revealed about his role in Infinity War.
There are going to be a lot of characters to keep track of when Avengers: Infinity War debuts on May 4. As preparation, you may want to watch a bunch of Marvel movies to make sure that you're up to date on every character that we've already met so that you can focus on everybody that's new when they start to arrive in Marvel's next adventure.