Only injecting this because I feel that only screaming, mentally unwell, and crying type roles in 'serious' films are usually considered for these lists but I think being fun and believable while telling a story in an ENTERTAINING manner (eg Julie Andrews in Sound of Music) is probably more difficult, as Shakespeare said about the jester/fool's role.
Reason being is I was in an after school Improv Club and noticed the light-hearted but believable style was not easy for people to do, inc me.
In fact I was THE worst at acting, though ppl in that school were too nice to say.
But the ONE thing I did well and that people praised me for, was a dramatic, serious depiction. But found that super easy...
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Take defeat as an urge to greater effort.
-Napoleon Hill