I sometimes “cast” actors as PCs and NPCs, borrowing some of their signature tics for the characterisation and possibly using pictures for handouts, particularly in media-based games. The Watch House was full of this, and I’d even worry about the budget of the imagined TV series when looking for regulars and guest stars. It was also amusing when we predicted a few actors’ future roles here and there - James McAvoy as a psychic, Emilia Fox as an Arthurian witch, Henry Cavill as a superhero based on his having auditioned to play Superman in 2005 before getting it in 2012.
Sometimes it happens the other way round as well. SFX magazine makes this a part of its regular Wishlist feature - often with specific roles in mind in adaptations, suggesting fan casting, but sometimes with a totally blank slate.
At the moment, rumours are flying around the geekosphere about potential casting for the new Star Wars trilogy. I just saw that Saoirse Ronan might be reading for a major role, which could be a hero (possibly Han and Leia’s daughter) or a villain. Who would I cast her as in Star Wars? Probably something that showcases her sad, wistful wise-beyond-her-years quality, like the leader of a lost people, although she’d also be great as a villain who appears to be one of the good guys...
It could be an Iron GM style ingredient - what role would a particular actor take in a given setting? Who would Benedict Cumberbatch play in the film of your classic fantasy game, or Steve Buscemi in your urban fantasy series, or Lena Headey in your space opera? Do you cast them very much to type or go against it? (I’d love to see Lena as captain of the next Enterprise, an idea I had after seeing her as Sarah Connor but before Cersei Lannister.)