The reaction is instant. In the space of time it took to get here, Shikamaru's typical unbothered-by-the-world expression had taken dominance, even hurried as he was. All traces of that disappear when Chouji drops down in front of him, however.

A moment to register - it is him and this is real - and then a wide smile breaks out over his face again. He jumps over to grab Chouji's shoulder. "It's so good to see you, man."

Other than the places where his friend has obviously stabbed himself in an attempt to break a genjutsu, that is.