And he insisted they can square the series with South Africa at Port Elizabeth by diverting all their focus on to the opposition.
Kellock, who is about to make the switch from Edinburgh to Glasgow, said: "We were not physical enough at Durban and we must be more in their faces.
"If you are in the changing room before banging your head off the wall and are so hyped up, you will probably drop off the aggression level when the real action starts.
"You don't need to run out on to the pitch with steam coming out of your ears.
"To my mind, it is a case of being level-headed to make sure you use your aggression the right way.
"If you are too wound up at the start of a game, you are more likely to give away silly penalties and that can be fatal."
Kellock, who has ousted veteran Scott Murray from Frank Hadden's line-up, continued: "Personally, I'll be trying to get around the park to make as many tackles as I can, being as physical as I can.
"The Boks are big, big guys, but if we front up and we hit them hard, give them a shock with the way we come into rucks and smash into mauls then we can get on the front foot and give our backs better ball."
Kellock believes his selection ahead of Murray represents a major milestone in his career.
He explained: "Five guys are vying for two starting slots - with a few others on the fringes of the squad.
"When I was listed on the bench for the first Test, the coach took me aside and assured me I was really in the mix.
"But it is not until you get the nod in front of guys like Scott that you know you are in the picture.
"The opportunity to play well from the start is a lot easier to take than with 20 minutes to go in a game.
"If you make one mistake when you're playing 20 minutes of rugby then that one mistake can be what's remembered, whereas throughout a game you might make a few mistakes but you have chances to do some good things as well."