Cassian laughs a little in spite of himself, disarmed by the sincerity at the same time that he's darkly amused by the phrasing. Good shadows.
"Suit yourself. I get off at the second stop, though." He's making his own observations, even as he says it: the look in those eyes, the loud clothes and quiet demeanor, the apparent lack of supervision. "Are you by yourself?" His tone is more quizzical than anything else, suggesting that if Will is by himself, he doesn't intend to do anything about it one way or another.