McCain seemed to suffer most from an apparent audio problem on the stage, which made it difficult for the candidates to hear the moderators' questions. Three times he had to ask for a question to be repeated; Giuliani also needed one of his questions re-asked.
In the end, it came back to Thompson, and his impact on the race.
Romney delivered the line of the debate with his analysis of the actor's entry onto the stage.
"Is this our sixth debate, I think, something like that?" Romney began. "This is a lot like 'Law & Order,' Senator. It has a huge cast, the series seems to go on forever, and Fred Thompson shows up at the end."
That notion, that Thompson had waited a long time to get into the race, led moderator Maria Bartiromo to ask the former senator how it felt to finally be here.
"I've got to admit, it was getting a little boring without me," Thompson said, "but I'm glad to be here now."
ABC News' Matt Stuart contributed to this report from Dearborn, Mich.