Anyone Whatsoever

In his introduction to "House Rule" (for reference, see here), Poul Anderson writes of his Old Phoenix inn:

"...to the best of my knowledge, nothing else is quite like this place, where you may meet anyone whatsoever."

Neil Gaiman's Inn of the Worlds' End, published later, is another venue where people from different universes meet and those who are real in one world may be mythical or fictional in another. In fact, Worlds' End fitted into an already existing fictional multiverse. Superman, published continuously since June 1938, never aged and  his stories were always set in the present. To account for this, it was editorially decided that earlier stories were set on Earth 2, current stories were set on Earth 1 and comics writers on Earth Prime mentally tuned into Earths 1 and 2. It got more complicated but that was the essence.

Arnold Schwarzenegger's Last Action Hero did for feature films what DC Comics had done for their superheroes: Jack Slater, real in one world, is played by Schwarzenegger in a film in another.

CS Lewis' "Forms of Things Unknown" begins with a quotation from Lewis' Perelandra:

"...that what was myth in one world might always be fact in some other."

And, in SM Sirling's Emberverse:

"'Our world is one of many through the cycles of the universe. Many...many iterations. Some very different, some much the same, some just different enough to be like an image seen in a distorted mirror. Deeds and persons and places echo from one to the other; sometimes what is dreams or tales in one is sober truth in the next.'"
-SM Stirling, The Sea Peoples (New York, 2017), Chapter Six, p. 89.