hard to think of someone with a high enough profile, perhaps Maggie Smith?
Oh yeah, Maggie Smith. HI5, bijou, you jewel, you nailed it!
As for Mrs. Benjamin Affleck, YOU are not worthy to fill Ms. Marple's shoes and Ms. Christie is rolling over in her grave as we speak.
Hollywood, in general, totally blows if you're looking for authenticity. No offense, Chuck Norris, Gary Sinise, Hollywood NeoCon, et.al.