Movie star Sean Penn felt sure his onscreen gay lover in new biopic Milk was a big boy - because James Franco wore a prosthetic penis for naked scenes.

The Spider-Man hunk strapped on a fake phallus for a nude pool scene in the film - about San Francisco's first gay Mayor Harvey Milk - because he didn't want to be completely exposed.

And even Penn was duped by the lifelike manhood.

Franco reveals, "This scene went on for a long time, like half the day, and it's getting old... and I go over to Sean and I guess he didn't know that I was wearing a prosthetic.

"I go, `Sean, you're such a great actor but you wouldn't do a scene like this if they asked you; you wouldn't dive into a pool naked.'

"And he said... `Well James, if I was built like you, I would.'

"A couple of weeks later we did this scene, where we're both dancing and we're naked, and we both have prosthetic penises. He finally put it together that I'm wearing, like, the Boogie Nights prosthetic."

- Sean Penn Photos
- James Franco Pictures

(This news article provided by World Entertainment News Network)