Don't mistake that serenity though for a soft touch.
Rennie might walk quietly but he carries a big stick. He is eloquent and can cut players down with a stare.
If players do not work hard enough in his squads, they will hear about it.
Rennie ensures his rugby teams have the basics sorted while fitness and attitude are non-negotiable.
He chipped out some of the old guard in the Chiefs this season as he and his staff installed their methods.
He, assistants Tom Coventry, Wayne Smith and Andrew Strawbridge all trained as schoolteachers and have a strong grasp on a range of instruction skills and group psychology.
Rennie and his staff got the Chiefs back into the zone for their semifinal win against the Crusaders.
"Dave would have put it on the young blokes," an old colleague observed.
"It is typical of how he can be. He would have told them cometh the hour, cometh the man and questioned whether they were making up numbers or were better than that."
Throughout his working and coaching life, Rennie has not differentiated in his approach with elite players and grassroots sporting associations.
The teacher and pub owner turned coach has that rare mix of common touch, humour and ability to connect with all players in his squad.
His guidance has not changed from the days when he had superstars such as Christian Cullen, Tana Umaga and Jonah Lomu in his rep team to his work with a local knockabout pickup side.
When he shifted to Manawatu his eye for talent development fixed on men such as Aaron Cruden, Andre Taylor and Aaron Smith while his ability to pull out game plans to suit his squad was also admired.
In his own Wellington playing days Rennie was a strong midfield back who had felt the patterns and flows of that pivotal area.
He also represented the Cook Islands, although he tries to laugh that off.
"I went back home for Christmas in 1989, all our family were there and the Cooks played a game in January and they tried to get me to play but I was thinking it was a bit early for me.
"Then they asked me to help out so I did and the next thing I know I am running at centre and the next thing I am playing.
"It was a lot of fun and I just had one game against a touring Australian province, that was about it really, it was nothing major."
Rennie has no idea if he earned a cap for the game but loved the experience. He never got another chance as the Cooks test schedule was infrequent.
"I had a kid assigned to me with a big bottle of water and he had to run on for me and make sure I was ok," he said.
"We played about 6pm when it was still bright but not ultra hot and the field was soft enough after rain in the summer."
While Rennie battled in those broiling conditions he ticked over 59 caps for Wellington in a six year career as he competed for selection against men like John Schuster, Marty Berry and Tana Umaga in midfield.
These days Rennie is encouraging Cruden and Sonny Bill Williams to use their gifts to unlock that congested area of the park. He has pared down Williams' broad portfolio and seen exceptional results while Cruden has been pushed to use his dazzling array of passes, feints and stutter moves.
The Chiefs coach sees many similarities between coaching and teaching.
"It is the same thing really, the kids are just a bit bigger," he said.
"When you teach, 99 per cent of the kids are good buggers and it is probably similar in this sport.
"We are lucky we get to focus on the subject that we are all passionate about from a teaching point of view.
"In teaching you need to be a people person and focus on developing young minds and attitudes and success in rugby is all about creating an environment too. It is sort of a progression from teaching to rugby I suppose. It is a nice fit."
Rennie taught fulltime for about seven years and then part time while Wellington rugby coach and publican of the Lonely Goatherd.
"That's like teaching too," he said. "They are bigger kids and you get a few ratbags but most of them are bloody good people."
At the end of 2002 when Wellington administrators decided Rennie had to move on, he was replaced by John Plumtree, his rival coach in tomorrow's Super 15 final in Hamilton.
Rennie is one of those men whose blood is really stirred by an involvement in rugby. It has barely left his system since he was born 48 years ago in Upper Hutt.
It is a passion plus.
Even when he was tipped out of Wellington he coached out the local under 21 side for three seasons because he loved the game and coaching.
"It is a lot about trying to make a difference and trying to develop their skills and when that happens it is really rewarding.
"Seeing some kids go on and play at the highest level gives you a buzz that you have been part of their progression.
"This job is pretty fickle though. You can be in one moment and out the next so you can't necessarily plan a life career in it but you have to enjoy it while you are there and the last few years have been very rewarding."
