they can.
directors are just shit when it comes to directing them.
you don't take a non-actor and make him spout lines. you gotta see what his natural style of expressing himself is, verbally and physically, and then build off of that.
there's a long tradition of directors doing that with any type of non-actors, and getting great performances.
so next time you see a fighter look like shit in a movie, he wasn't directed right, the script wasn't written with him in mind, and MOST IMPORTANTLY, he wasn't filmed in a way that would take advantage of his natural expression.
here's chuck looking GOOD in a mock trailer. had the director been better, he would have looked fucking awesome. but I like it as it is, it doesn't make any excuses, doesn't use the Chuck "character", but builds off of his natural persona and just gets something real on screen.
contrast it with randy's monologue in the expendables, I thought he was taking a shit while talking. which he probably was.
http://www.youtube.com/watch?v=lBx3-A3mLD8&feature=topvideos
God damn chuck is a bad actor. 
dude, that's the fucking point.
he's not "acting", he's being a stone faced bad-ass/moron in court.
if you don't see that as an awesome 80s movie, I want to bash your face in.
actors can't fight.
can't take acting classes if you're in the gym 8 hours a day.
Kos needs an Oscar for some of the bullshit he has pulled in the Octagon.