Barnes only appeared after Titanic hit the iceberg.
He was ordered to get the First Class women and children on deck without causing any panic. When he found Cal & Lovejoy he ensured him there was no emergency, but Cal bit back: "Yes there is, I have been robbed!" He was talking about the Heart of the Ocean, which Cal wanted to blame Jack for stealing. Earlier that day, Jack had "borrowed" the coat of A. L. Ryerson and now Lovejoy was going to slip the diamond in a pocket of the coat. Barnes went to get the Master-at-Arms after an order from Lovejoy and a "moron" shout-out from Cal.
Soon after, Cal, Ruth, Barnes, Steward 1 & King were in the room. King was looking approvingly at Jack's drawings, but Cal snatched them out of his hands and warned Barnes to not touch anything. Meanwhile, Lovejoy set the trap by slipping the diamond in the pocket of the coat Jack was wearing. The trap worked perfectly: Barnes fetched a reportedly stolen diamond out of a reportedly stolen coat. Jack is taken away for theft.
Soon after, Barnes returns with Captain's orders to come up to the deck with lifebelts. Cal tries to send him away again, but this time Barnes proceeds. He takes the lifejackets and suggests Cal & Rose to dress warmly because it's cold outside. He tries to comfort Rose that it's just a precaution.
Barnes is never seen again and presumably died in the sinking.