Could a studio realistically disguise until the premiere the presence of an A-list actor in a small but non-trivial part in a movie they would not be expected to be involved in? Presumably if they're spotted by anyone working at the studio not under an NDA, the game's up. But maybe people working in the industry have a way around this? Would a studio ever want to keep casting details completely secret, for a start? Is this kind of detail typically at the Director's discretion or the studio's?
The question is prompted an example from the Marvel comics canon, where in Agents of SHIELD (I doubt this is really a spoiler by now but just in case some people are behind):
Agent Skye's birth name is Daisy Johnson, who in the comic canon has telekinetic powers much like might be expected from a Mutant (although she is explicitly described as not a Mutant in the same sense as the X-Men) and is involved in SHIELD's management of Mutant extremists, for example in apprehending Magneto. The upcoming Age of Ultron features characters who most definitely are Mutants, so there are two counts where the Avengers continuity and the X-Men continuity co-exist. This raises the possibility of major X-Men characters like Magneto or Professor Xavier showing up in Avengers-themed movies and vice versa, contractual issues between studios aside. Possibility in the sense that it amuses my fantasist's mind, and not because I expect them to ever actually do so.