Mistaking the argument and the person

Ever heard of the sayings "don't attack the messenger, attack the message" or "don't hate the player, hate the game?"  It's the same concept when it comes to judging an argument based on the arguer. People, when listening to an argument, often decides their sides based on the person presenting the argument. An argument should be based on it's premises and claims rather than basing it on the "messenger."

An example would be with celebrity endorsed advertisement and products. One may already form an opinion based on the celebrity in the piece of advertisement.
In the Proactive ads, they use a lot of celebrities ranging from Katy Perry to Justin Bieber to Avril Lavigne.
Someone might mistake the person for the argument because of their opinion on the celebrity. They might think that "oh celebrities have enough money to go out and buy more expensive and effective products than proactive."

Basically in mistaking the person for the argument is when someone focuses more on the person than the actual argument.