I don't give a shit about Agatha Christie, but Kenneth Branaugh is an amazing actor,and director, and writer, and whatever else he does. In fact, he holds the Oscars record for most nominations in separate categories. He has been nominated for Best Something in 7 different categories (Actor, Director, Live Action Short Film, Adapted Screenplay, Supporting Actor, Original Screenplay, Picture).
I wouldn't call him an overactor, but he is certainly capable of acting LARGE when the role calls for it, and he is also capable of incredible subtlety when the role calls for it. He has a lot of range, which most actors no longer do.
And yes, EB, his Hamlet is incredible.