Holmes is impressed at how Stapleton's so cool under pressure.
Yeah, he's cool—it's not like Holmes can prove any of his ideas in a court of law yet.
(This is just like that Columbo episode where he knows the plumber's lying but—oh, sorry, we digress…)
Holmes believes that Laura Lyons will be the key to this stage of the case.
Watson brings Holmes to Baskerville Hall, and Sir Henry welcomes him.