Lithgow is, undeniably, a much bigger name than Rylance, at least in his native U.S.. (Although it’s kind of weird that they’re looking to cast an American actor in the role of one of the iconic British wizards;

Hollywood has a not-so-mysterious way of casting who they want for roles in films and television shows with major IPs. There is usually a breadcrumb trail that you can trace back to a few previous projects that will help you understand why a choice was made for a particular actor.

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