Seamus
Overall, it's obvious to anyone who knows him that Seamus is not in the greatest of moods. He misses Dean. His black powder was confiscated. First years were supposedly left on the train. Harry, Ron, and Hermione are gone, plus all of the other Muggleborns. He's sore that McGonagall isn't Headmistress (and told her as much when he saw her).
Seamus looked less than pleased to be called up after that kind of welcoming speech, as he doesn't want to be thought of as endorsing any of these changes. He stood with his hands in his pockets until dismissed.
Having been present for six years of prefect speeches, he's actually pretty okay with stepping into the role. He made sure to shake all of their wee little hands with a smile, introducing himself personally. He's glad that Jack and Rachel are present too, and would have looked to them after giving the routine introduction with Parvati, in case anything was missed.
I'm guessing they all went with them back to the Gryffindor common room, pointing out bathrooms and classrooms along the way, introduced them to the Fat Lady, assured them that they had the best head of house, etc. Once they were safely inside the common room, he would have been willing to talk about what happened on the train.
Hi Neville. Should I assume there was a night of talking?