Facinelli has played Cullen family patriarch Carlisle since the first Twilight film, and his role as a vegetarian vampire leading a troop of like-minded undead has always been paternal, even though the Cullens aren't related in the story.
In this final film, the Cullens are being attacked by the vampire Powers That Be, the Volturi, for harbouring a child that is half-vampire, half-mortal -- the result of the relationship between Bella and Edward, the film's two leads played by real-life couple Kristen Stewart and Robert Pattinson.
Both have had a rocky time of late after it was revealed that Kristen had cheated on her boyfriend, but seem to be putting on a united front as their promotional duties for Twilight kick off.
Peter is unsurprisingly keeping schtum about the scandal, curtly saying: "I don't have the guard up when it comes to promoting this film, because it has nothing to do with the film. This movie to me is the same as any other in the franchise.
"Fans are coming to see Edward and Bella, and what goes on in my life, theirs, anybody's, doesn't affect the performances that were given or the fans' perception of the story."
Peter himself is newly single, having recently split from wife of 11 years, Beverly Hills 90210 star Jennie Garth. "I don't discuss that aspect of my life, out of respect to my children."
The couple are now co-parenting their three daughters, the youngest of whom is five. "The baby is the entertainer of the family. She has some interest in the movie business, the most of the three. The eldest loves Twilight. Luca is 15 and gets to go to the premieres with me. She's in high school."
It must be odd that his daughter is part of the demographic that swoons over Carlisle?
"Yes, it's a bit weird, so I don't really like to think of it that way. People look at you one way or another, and this has been a new level of fame and fandom. It's been a great ride, with very supportive and giving fans. It's been very humbling; I've been working for 17 years and have had some success, but being a part of a cultural phenomenon is rare in any actor's career, and I'm grateful to have experienced it."
Breaking Dawn is the most adult of Stephenie Meyer's series, and sees Bella and Edward getting married and conceiving the aforementioned 'immortal child', but will this final instalment be true to the beloved novel? "It's been more than a year since we finished filming, so it's hard to remember exactly what went down," explains Peter.
The final book was split into two films, but shot together out of sequence. "We knew it would be two separate movies, but weren't sure what would be going where. I haven't seen it, so I'm excited.
"However, I do know it's loyal to the books, with a couple of surprises. I love those little golden nuggets -- it's special for me to have those moments of surprise for the fans. The content in this one is a little more mature, but the filming experience was similar, apart from the fact that there are lots of different covens, so a lot of new actors -- there's even an Irish one."
Carlisle calls in favours from peaceful vampire covens all over the world to help defend his family against the Volturi. "Yeah, we go through his Rolodex of vampire friends. This one is a lot more supernatural than the other films: a lot more greenscreen moments, special effects, and everyone has a special power, which I enjoyed. I'm really looking forward to the fans' reaction."
The child the characters are going out on a limb to protect is played by Mackenzie Foy (7). "The kid who plays little Renesmee was fantastic; I was impressed. She's going to be a huge star.
"Working with younger actors, it can be hard to keep their attention. She had great stamina, and great connections with the other actors. She had to wear contact lenses all the time too, which wasn't easy for a little girl. She was amazing."
Now that the saga is winding down, Peter is focusing on his production company and his role in hit American TV show Nurse Jackie, which is entering its fifth season.
However while he's not ruling out a return to Twilight, he's playing it cool. "I don't know if there will be a prequel or anything like that," he says. "That's up to Stephenie Meyer."