Okay I know this is old news to most people but I have almost never watched a "Smallville" episode on television. But I've been collecting it on DVD and just started watching season 5. Its funny that James's first real episode discounting a cameo comes in a "Buffy" parody. I just listened to the commentary on the episode and its funny and interesting.
The producers apparently didn't care much for the episode though it got rave response from fans many of whom were also "Buffy" fans. The choice to have Chloe narrating the episode was something they came up with after. I loved the line where Chloe talks about the blond cheerleader turned vampire and says, "We'll call her- Buffy Saunders" and then mentions that that might not be her real name but she's making up names to protect the innocent. That part nicely makes it funny rather than expecting us to believe a blond cheerleader becomes a vampire and just happens to be named Buffy and 'Saunders rhymes with Summers'.
They didn't care for it only because they felt it was outside the "Smallville" cosmology and was a bit forced to bring vampires into it.
In the commentary they joked that they considered having Profesor Milton Fine meet the vampires and get turned but that might seem a bit of a rehash. Then in the scene where Fine (Braniac) does a terminator maneuver and turns his hand into a blade to impale a guy, one of them joked, "We felt we needed a Spike in this episode".
But jokes aside, they talked about what a great actor Marsters is and one of them even said exactly what Joss Whedon once said. They never intended the character to be played quite how James was playing him and he was making the character far more than he was on paper. He was saying the words but the body language, the pacing of the words, the personality wasn't how they had pictured Braniac. But then they realized, its better than what they pictured. Some of the way Marsters talked showed he was literally an alien to this setting, to this planet. His speech patterns were different than other people's in the show and Marsters is the one that brought that quality to it. One comment from one of the show's creators is that he realized what a great actor Marsters is and sometimes you just have to shut up and not tell a great actor how to act. You let him show you. He also mentioned how Michael Rosenbaum (Lex Luthor) is a good actor but it was a joy to see him rise from good to great thanks to working with Marsters and that's one of the marks of a great actor, he knows how to coax better performances out of those around him without overwhelming them.