The cast of the indie film “The House Across the Street” ­— including Jessica Sonneborn, Ethan Embry (of “Can’t Hardly Wait” fame), and Mike Malvesti (who had bit parts in local movies such as “The Town” and “Moonrise Kingdom”) — hung out on the South Shore Saturday afternoon with their director Arthur Luhn. “The House Across the Street” is a horror-thriller. The tagline on the film is: “What you don’t know won’t hurt you; but what you know might kill you.” That would be scarier if everyone didn’t seem so happy in this picture.