That might sound all well and good to some. Except that Miss Marple is generally expected to look like THIS:
Now, I'm not gonna say Ms. Garner isn't a fetching sort of gal, and I'm not gonna say I own a copy of 13 Going on 30 due to her charms (that would be ridiculous, if it weren't true). But I've held a fond affection for A) Christie mysteries and B) the series of Marple films that starred Margaret Rutherford for some time, and I don't see how this makes sense.
I know Disney wants to adapt the stories for a "younger audience", but it's not like little kids don't like old people. Why wouldn't you cast a Judi Dench or a Helen Mirren or a Stephen Fry in drag? It's not Miss Marple if it's Jennifer Garner, is it?
I don't know, but I'm throwing my fat bottom at the next James Dean biopic they make. This gives even the worst casting ideas hope. Just picture this as Jennifer Garner, then get back to me.
(BTW, that theme music rocks.)