How does Macbeth act when the other thanes arrive at his castle?

It's act 2, scene 2, and the dastardly deed has been done—Duncan lies brutally murdered in his bed. The world outside doesn't know about it yet, but that's about to change. After engaging in a spot of raillery with a hungover porter, two thanes—Macduff and Lennox—enter the castle, hoping to speak with Duncan. For obvious reasons that's not going to happen. Instead, the two thanes initially encounter Macbeth.
Straight away, we sense that something's not quite right— Macbeth speaks in short, clipped sentences, indicating a profound unease at Duncan's murder. Although Macbeth successfully managed to appear as an innocent flower when Duncan arrived, now that the dirty deed has been done, his mind is beset with dark doubts and intimations of doom.
In the presence of Lennox and Macduff, it's difficult for him to keep up the pretense that all is well, so Macbeth doesn't want to say too much for fear of exposing his guilt. Yet his behavior speaks volumes. While everyone else is running round like a headless chicken screaming blue murder, Macbeth appears suspiciously calm. This is due in no small measure to his increasingly pessimistic outlook on life, which induces a kind of stoic acceptance of what's to come:

[F]or from this instant There’s nothing serious in mortality. All is but toys. Renown and grace is dead. The wine of life is drawn, and the mere lees Is left this vault to brag of.