Can Marky Mark actually act or was he cast because he basically is Dirk?
This evergreen topic came up just last night on a podcast I was listening to, where essentially they were talking about "actors" versus "movie stars".
I don't want to use that terminology as I think it can sound dismissive, but how I put it is that movies typically have actors who attack the roles in two different ways:
1. Charm offensives: these actors have honed in on the aspects of themselves that audiences connect with. They are very good at playing that up on film, mostly playing versions of themselves as they experience different emotions. See: Tom Cruise, Bruce Willis, Mark Wahlberg, etc.
2. Espionage: these actors do their research and sink into a character. They are very good at becoming whatever they need to be for the master plan, sometimes leading them to be overlooked. They can be the element that delivers the killing blow, or the one that plays their part but the mission (film) fails because they aren't given the right orders. See: Gary Oldman, Johnny Depp, Daniel Day Lewis, etc.
Most movies benefit from having both types of actors. If all your actors are spies, the film sometimes doesn't have enough flash...it really requires masterful writing and direction to keep it gripping for a wide audience. If all your actors are trying to launch charm offensives, the film usually doesn't feel like it has enough room to breathe. There's no space for narrative and clever direction because everyone's competing for screentime.
Boogie Nights is a great film because it has actors of both types and it knows how to use them very well. Wahlberg's particular blend of "gee shucks" mixed with some darkness is perfect for the role, and he bounces well off other charm offensives like Reynolds and Reilly. Then you've got spies like Moore and Hoffman gluing the movie together by being exactly what they need to be for that part. PTA really has a kind of genius for casting.