In "Devil's Trap" when Bobby Singer is talking to Sam about the Key Of Solomon, it goes to show Bobby on the left and Dean in the back ground on the right. You can see Jensen wording his next lines. There is no reason for him to be mumbling in the episode.

If you go to "Folsom Prison Blues" in Season 2 when Dean is playing poker for cigarettes, he stands up and says "Hey Fella's" And if you look at Jared he is mouthing Dean's lines.

In the end of "Bad Day at Black Rock." In the end when Dean spazes about Bella and yells, "Son Of A B**ch" Jared starts to laugh and turns away cause he doesn't know what else to do.

Jared himself confirmed these actions at Chicago Con.