There are no details at all so far about the role McBride would play, but the situation has apparently come about through director Scott and McBride bonding over their mutual love of film.
It is easy to make the obvious cracks about the hapless egocentric drug addled sweary slacker pratfalling that McBride might bring to the Alien universe. But this could also be his opportunity to display a significant broadening of his range, in something like the way his fellow Seth Rogen (This Is The End) cohort Jonah Hill has done recently in Moneyball (2011) and The Wolf Of Wall Street (2013).
Whatever the character, he will somehow fit into a story that sees the crew of the colony ship Covenant heading for a remote planet at the edge of the galaxy. They discover what they think is an uncharted paradise, but is actually a dark, dangerous world, the sole inhabitant of which is the android David (Fassbender), survivor of the doomed Prometheus expedition.
Alien: Covenant is set to start shooting in Australia next month for a 2017 release.