Patrick Roach actually stood shirtless in the weather temperature of -35 Celsius on the first day of filming. According to production notes, Roach never complained once.
The 4/20 event on Parliament Hill, Ottawa, took place on April 20, 2013. Ottawa police were recruited as security and around 50 extras along with the film crew surrounded to protect Robb Wells, Mike Smith and John Paul Tremblay. The three actors were unnoticed by over 10,000 attendees until onlookers started approaching them at the last minute.
Robb Wells did all of his own driving including the bus.