It depends on what you mean by "mention."
It is perfectly fine to have one of your characters mention somebody else's character in dialogue in a context that makes it clear that they are talking about a fictional character who is NOT part of your own story. An example would be "You know that Rick Deckard guy from Blade Runner? It's like what he did in..." You can also have it in a description so long as it is clear that it is a similar context: "It looked just like that gun from Aliens."
It is NOT okay to mention them as though they are characters in your story. That you can't do without permission from the rights holder. You can sometimes get away with a description that doesn't include a name and could be anybody, but those in the know will pick up that it's a reference to a specific character, but that has to be done very carefully.
(NOTE: Please don't reply asking me for clarification - I'm still months behind on my professional work, and the only reason I'm here right now is that I had an extra ten minutes I wasn't expecting. I have to disappear again.)