This is the movie where Daniel Radcliffe stares at a musty Sports Illustrated swimsuit issue and then grows a stiff and mighty erection, which Paul Dano then uses as a compass to point the way back to civilisation.
Daniel Radcliffe answer the question
Vulture journalist Kyle Buchanan then writes, since I’m a responsible journalist who’s curious about low-budget practical effects, I asked Radcliffe about his erection. How’d they manage that wiggly piece of equipment, which gets plenty of close-ups as it points the way home through Radcliffe’s pants?
How did the mechanical boner rig work? Radcliffe said it went through several iterations. “One of the pieces that I had on was just a wooden — wow, I can’t believe I’m telling you this — it was like the end of a broom. It was all just like one line, and too smooth. So that was the one note that I gave, and they took it and ran with it.” Radcliffe started laughing. “Is this too much information?”
Read the whole interview with Daniel Radcliffe by the responsible journalist Kyle Buchanan that asks the obvious questions here.