Terminator 2 and X-Files star Patrick has been cast as Jackson, the father of Manganiello's werewolf Alcide.
"I just worked with my 'daddy' till 6am on Saturday morning and we're having a blast," Manganiello told TV Guide.
The actor promised that the storyline involving Alcide and Jackson's reunion will be "creepy" and "dark".
"It gives so much back story into why Alcide hates being a werewolf," he explained. "It is so great!"
True Blood star Todd Lowe (Terry Bellefleur) recently hinted that the HBO drama will return for a sixth season, while discussing star Anna Paquin's pregnancy.
"I think she is due right before we start [filming] the sixth season," said Lowe. "It would be hard [to have her character Sookie fall pregnant too] because of the timeline. The timeline is crunched, you know."
True Blood airs on HBO in the US and on FX in the UK.